How does light affect the universe?

How does light affect the universe?

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When light is emitted by for example a star, that star loses energy - which causes it to reduce its gravity. Then that energy begins a journey for potentially billions of years, until it reaches some other object.

When that light reaches a surface, such as another star or galaxy, it will give that energy to the destination star in the form of heat. This causes the receiver to increase its energy, in turn restoring a sort of balance. It also causes the receiver to emit a minute amount of more light again, almost like a reflection.

It will also excert pressure on the receiving surface once it reaches its destination, be it a star, a rock or anything else.

But while that light is travelling through space, its energy is "unavailable" to the rest of the universe. Naturally I ask the following question:

Will light cause gravity, while it is traveling?

Every single star emits light in every direction, and will eventually reach every other star in the universe. At any single point in the universe, there must be a continous ray of light coming from every single other star in the universe, that has a direct path to that point. Given that all stars on the sky is sending photons that reaches every square centimeter of the earth surface, the amount of pressure should sum up to be quite large.

Is the amount of pressure really neglible, given that every single atom on any surface is receiving light from every single lightsource on the sky?

Based on a calculation found at the sun will during its lifetime emit 0.034 % of its total mass as energy. Assuming the sun is average, and that there are about 10^24 stars in the universe, and all of these stars on average are half way through their lifetime, there should be energy amounting to the gravity of about 1.7*10^22 suns distributed throughout the universe.

Old question, but I'll address something that hasn't been brought up by the previous answers.

Photons $simeq$ CMB photons (to first order)

As the others has already said: yes, light has energy and hence it gravitates. The bulk of photons that permeate the Universe isn't of stellar origin, though, but is in fact the cosmic microwave background, the energy density of which several orders of magnitude larger than other photons, as seen in the graph from this answer to "Number density of CMB photons". In terms of number density, there are 4-500 photons per cm$^3$.

Space is big and isotropic

Since CMB photons are isotropically distributed, the ever-so-small radiation pressure is equal in all directions, and hence cancels out. And although we're all the time bombarded by both CMB photons and stellar photons, space is so mind-bogglingly big (D. Adams, 1978) that if you consider a random photon in the Universe, the probability of it hitting anything at all is negligible. Roughly 90% of the CMB photons have traveled for 13.8 billion years without hitting anything; the remaining 10% interacted with the free electrons that were released after reionization, but weren't absorbed, just polarized, and by far most of these interactions took place shortly after reionization; by now, the Universe has simply expanded too much.

Photons are redshifted

Although there is energy in photons, and hence they add to gravitation, first of all they're homogeneously distributed in the Universe (and thus pulls equally in all directions), and second their energy density is negligible compared to baryons ("normal matter" like gas, stars, and planets), dark matter, and dark energy. In fact, their relative densities are ${ ho_mathrm{bar}, ho_mathrm{DM}, ho_mathrm{DE}, ho_mathrm{phot}}/ ho_mathrm{total} = {0.05,0.27,0.68,10^{-4}}$. But this was not always the case. As the Universe expands and new space is created, the density of matter decreases as $1/a^3$, where $a$ is the scale factor ("size") of the Universe. The same is true for photons, but since additionally they're redshifted proportionally to $a$, their energy density decreases as $1/a^4$. That means that as you go back in time, the relative contribution of photons to the energy budget increases, and in fact until the Universe was 47,000 years old, its dynamics was dominated by radiation.

Light causes gravity while travelling, a clear yes, by Einstein's famous mass-energy equvalence. (Compare this discussion on StackExchange.)

The gravitational pull of light is negligible to other mass in large scale. Only a small fraction of mass of a star is transformed into light during its lifetime, and only a small part of the ordinary matter has ever been a star. A fraction of the ordinary (standard model particles) matter consists of neutrinos (neutrinos and electrons are leptons). The baryonic matter consists mainly of hydrogen and some helium (nuclei) formed shortly after the big bang.

A small fraction of mass of a star consists of photons, travalling out of the star. This travel can take millions of years.

The effect of light on asteroids is not negligible, but it's not the gravitational pull. It' mainly the YORP effect. Dust is also affected by light.

Yes, light gravitates. The gravitational charge is energy. Well, gravity is a spin-2 force, so you really have momentum and stress as well, but they are analogous to a generalization of electric current.

In general, anything that contributes to the stress-energy tensor will have some gravitational effect, and light does that, having both an energy density and putting a pressure in the direction of propagation.

But while that light is travelling through space, its energy is "unavailable" to the rest of the universe.

Not quite. It still gravitates. However, the radiation-dominated era was before about 50k years after the Big Bang, but it is long past. Today the gravitational effect of radiation is cosmologically negligible. We live in a transition between matter-dominated and dark-energy-dominated eras.

Given that all stars on the sky is sending photons that reaches every square centimeter of the earth surface, the amount of pressure should sum up to be quite large.

The light pressure on any surface is proportional to the light energy density incident on it. Thus we can check this line of reasoning directly by observing that the sky is dark at night.

Why it is dark at night is probably deserves its own question (cf. also Olbers' paradox), but it is pretty clear that it is in fact quite small. To be fair, we should check more than the visible range, but even so the sky is pretty dark. Thus on average, light pressure is very small.

We have the privilege of being close to a star, but even during the day, the light pressure due to the Sun is on the order of micropascals.

… there should be energy amounting to the gravity of about 1.7*10^22 suns distributed throughout the universe.

And this is a tiny amount. As you just said, this is the equivalent of about 0.034% of the total mass of stars in the universe, which is in turn constitute but a fraction of the matter in the universe. So why you are surprised that its effect is negligible? It's literally thousands of times less than the uncertainty in the measurements of the amount of matter in the universe.

Dispersion of Light

Look up into the rainy sky! What do you see? Well, if its just rained and the sun is once again shining, chances are you see a rainbow. Always a lovely sight isn’t it? But why is it that after a rainstorm, the air seems to catch the light in just the right way to produce this magnificent natural phenomenon? Much like stars, galaxies, and the flight of a bumblebee, some complicated physics underlie this beautiful act of nature. For starters, this effect, where light is broken into the visible spectrum of colors, is known as the Dispersion of Light. Another name for it is the prismatic effect, since the effect is the same as if one looked at light through a prism.

To put it simply, light is transmitted on several different frequencies or wavelengths. What we know as “color” is in reality the visible wavelengths of light, all of which travel at different speeds through different media. In other words, light moves at different speed through the vacuum of space than it does through air, water, glass or crystal. And when it comes into contact with a different medium, the different color wavelengths are refracted at different angles. Those frequencies which travel faster are refracted at a lower angle while those that travel slower are refracted at a sharper angle. In other words, they are dispersed based on their frequency and wavelength, as well as the materials Index of Refraction (i.e. how sharply it refracts light).

The overall effect of this – different frequencies of light being refracted at different angles as they pass through a medium – is that they appear as a spectrum of color to the naked eye. In the case of the rainbow, this occurs as a result of light passing through air that is saturated with water. Sunlight is often referred to as “white light” since it is a combination of all the visible colors. However, when the light strikes the water molecules, which have a stronger index of refraction than air, it disperses into the visible spectrum, thus creating the illusion of a colored arc in the sky.

Now consider a window pane and a prism. When light passes through glass that has parallel sides, the light will return in the same direction that it entered the material. But if the material is shaped like a prism, the angles for each color will be exaggerated, and the colors will be displayed as a spectrum of light. Red, since it has the longest wavelength (700 nanometers) appears at the top of the spectrum, being refracted the least. It is followed shortly thereafter by Orange, Yellow, Green, Blue, Indigo and Violet (or ROY G. GIV, as some like to say). These colors, it should be noted, do not appear as perfectly distinct, but blend at the edges. It is only through ongoing experimentation and measurement that scientists were able to determine the distinct colors and their particular frequencies/wavelengths.

We have written many articles about dispersion of light for Universe Today. Here’s an article about the refractor telescope, and here’s an article about visible light.

If you’d like more info on the dispersion of light, check out these articles:
dispersion of Light by Prisms
Q & A: Dispersion of Light

We’ve also recorded an episode of Astronomy Cast all about the Hubble Space Telescope. Listen here, Episode 88: The Hubble Space Telescope.

Ask Ethan: Can Dark Matter Really Explain The Universe’s Structure?

The formation of cosmic structure, on both large scales and small scales, is highly dependent on how . [+] dark matter and normal matter interact, as well as the initial density fluctuations that have their origin in quantum physics. The structures that arise, including galaxy clusters and larger-scale filaments, are indisputable consequences of dark matter.

Illustris Collaboration / Illustris Simulation

One of the most puzzling components of the Universe has to be dark matter. Although we have extraordinary astrophysical evidence that the normal matter in the Universe — the stuff made out of known particles in the Standard Model — cannot account for the majority of the gravitational effects we observe, all of that evidence is indirect. We still have yet to obtain a shred of repeatable, verifiable direct evidence for whatever particle might be responsible for dark matter. The total evidence places very tight constraints on any non-gravitational interactions that dark matter might possess. But if dark matter only interacts via the gravitational force, can it really explain the Universe’s structure? That’s what Patreon supporter Dr. Laird Whitehill wants to know, asking:

“If dark matter particles don’t interact and the only force that governs their motion is gravity, how do dark matter particles coalesce into a cloud? [In other words,] why aren’t all the particles hyperbolic?”

This is a very deep question, and the answer takes us deep into the heart of how gravity works in the Universe. Let’s start in our own backyard.

Within our Solar System, the gravitational influence of the Sun has a dominant effect on all the . [+] masses that come close to it. The Sun represents 99.8% of the mass of our Solar System, and is the reason that all objects we've discovered have their orbits fall into one of four categories: circular, elliptical, parabolic or hyperbolic.

Here in our Solar System, over 99.8% of the mass exists in just one central location: our Sun. If any other mass comes close enough to be significantly influenced by the Sun’s gravitation, there are only four possible trajectories it can take on.

  1. It can make an elliptical orbit around the Sun, which it will always do if it’s gravitationally bound.
  2. It can make a circular orbit around the Sun, which is also gravitationally bound but has a special set of orbital parameters.
  3. It can make a parabolic orbit around the Sun, which it does if it’s right on the border of being gravitationally bound vs. being unbound.
  4. Or it can make a hyperbolic orbit, which is what it will always make if it’s gravitationally unbound.

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Objects that come into our Solar System from outside of it — interstellar interlopers such as ‘Oumuamua or Borisov — will always make a hyperbolic orbit so long as they’re only influenced by the Sun’s (and not any of the other objects in the Solar System’s) gravitation.

The most eccentric natural object ever discovered in our Solar System, 2I/Borisov is just passing . [+] through. In early December of 2019, it made its closest approaches to both the Sun and Earth, passing interior to the orbit of Mars. Borisov is now long gone, on its way back out of the Solar System on a hyperbolic orbit.

Casey M. Lisse, presentation slides (2019), private communication

That’s because gravity is what we call a conservative force: objects that only interact gravitationally will enter a region of space with the same speed and same kinetic energy that they’ll leave it with. Gravity will only change the object’s trajectory, not its speed or its energy both of those quantities are conserved, as neither energy nor momentum is either liberated or lost by the system.

Although we’ve observed this to be true in a great many instances — both inside and outside of our Solar System — it’s exactly theoretically true in Newtonian gravity, and would be exactly true in General Relativity if you were willing to ignore the minuscule amount of energy lost due to gravitational waves. Which means that any object that only interacts gravitationally, including a lone dark matter particle, would enter the Solar System at a particular speed, come close to the Sun and reach a maximum speed, would be redirected by gravity, and would exit the Solar System at the exact same speed (but a different direction) compared to what it entered with.

This schematic diagram of our solar system shows the dramatic path of the object initially . [+] designated A/2017 U1 (dashed line) as it crossed the plane of the planets (known as the ecliptic), and then turned and headed back out. This object is now known to have an interstellar origin, and was named 'Oumuamua. Its hyperbolic orbit arises from the Newtonian force law, and it leaves at the same speed that it entered our Solar System at.

Brooks Bays / SOEST Publication Services / UH Institute for Astronomy

The reason that normal matter forms the complex structures that we see, structures like galaxies, star clusters, individual Solar Systems and other “clumps” of matter, is because it can experience these non-gravitational interactions. Through the electromagnetic and nuclear forces, normal matter can do all of the following:

  • experience “sticky” inelastic collisions, where two or more particles bind together to form a composite particle,
  • interact with radiation, where they can either radiate energy away (in the form of heat) or absorb radiation, changing its kinetic energy and its momentum,
  • and can efficiently dissipate energy, enabling a type of gravitational collapse that dark matter cannot undergo.

Whereas, in an unchanging system, a dark matter particle that falls in at a certain speed would inevitably exit at the same speed (and radius) that it entered at, a particle made of normal matter could interact in a non-gravitational way with all the other particles of normal matter and radiation inside. In general, it will collide with those particles, transferring energy between them, leading to the production of radiation, and creating a more tightly bound final state than the initial state.

While the normal matter within a bound structure, like a galaxy, will collide, interact, and . [+] dissipate energy, the dark matter cannot do such a thing. As a result, the normal matter coalesces in the center, producing a small, matter-rich disk with spiral arms, stars, planets, and other very dense structures, while the dark matter remains in a large, diffuse halo without such small-scale structures.

Normal matter, because it can dissipate its energy and momentum in a way that dark matter can’t, can easily form bound, collapsed structures. Dark matter, on the other hand, cannot. If you only have gravitational interactions when you fall into an established, unchanging structure, you’ll leave with the same properties you entered with.

But the Universe isn’t truly an established, unchanging place, and that changes the story dramatically. In particular, there are two phenomena that we need to pay attention to, because they both play important roles.

  1. The Universe is not static and unchanging, but rather expanding over time.
  2. The structures within the Universe aren’t static and unchanging, but rather undergo gravitational growth over time.

These two facts each, on their own, can alter the fate of a dark matter particle that comes under the influence of a massive structure that it happens to encounter.

While matter (both normal and dark) and radiation become less dense as the Universe expands owing to . [+] its increasing volume, dark energy is a form of energy inherent to space itself. As new space gets created in the expanding Universe, the dark energy density remains constant. Our Universe contains numerous species of matter and radiation, including both normal matter and dark matter, and also contains a dose of dark energy as well.

E. Siegel / Beyond The Galaxy

1.) The expanding Universe. The fact that the Universe is expanding does a number of important things. It reduces the number density of particles, because it increases the volume of the Universe while leaving the total mass the same. It causes the wavelength of radiation to redshift, because the distance between any two arbitrary points in the Universe — even the two points that define what a “wavelength” is for an individual photon — stretches over time, lengthening its wavelength and bringing it to progressively lower energies.

Well, massive particles, even dark matter particles, are also affected by the expanding Universe. They aren’t defined by a wavelength the way that photons are, but do have a certain kinetic energy at any given moment in time. Over time, as the Universe expands, that kinetic energy will drop, lowering their velocity relative to any nearby observer as the Universe expands.

Here’s how you can picture it.

This simplified animation shows how light redshifts and how distances between unbound objects change . [+] over time in the expanding Universe. Note that the objects start off closer than the amount of time it takes light to travel between them, the light redshifts due to the expansion of space, and the two galaxies wind up much farther apart than the light-travel path taken by the photon exchanged between them. If it were a particle instead of a photon, it wouldn't redshift, but it would still lose kinetic energy.

Imagine that you’ve got a particle moving through space, from point A (where it starts off) to point B (which is where it’ll wind up). If space were unchanging and not expanding, and there were no gravity, then whatever speed it started out having at point A would be the same as the arrival speed at point B.

But space is expanding. When the particle leaves point A, it has a certain speed, where speed is defined as a distance over a time. As the Universe expands, the distance between point A and point B also expands, meaning that the distance increases over time. The particle itself, over time, traverses a smaller percentage of the distance separating A from B as time goes on. Therefore, the particle moves towards B at a slower pace near the end of its journey than near the beginning of its journey.

This applies even as a dark matter particle approaches and falls into a large gravitational structure, like a galaxy or a galaxy cluster. From the time it begins to fall into a structure to the time it would reach the other side and be ready to exit back out again, the expansion of the Universe has lowered its speed, meaning that an infalling particle that was only slightly gravitationally unbound when it first encountered a structure can become slightly gravitationally bound due to the expanding Universe.

The growth of the cosmic web and the large-scale structure in the Universe, shown here with the . [+] expansion itself scaled out, results in the Universe becoming more clustered and clumpier as time goes on. Initially small density fluctuations will grow to form a cosmic web with great voids separating them, as structures with more mass than others will preferentially attract all of the surround masses.

2.) Gravitational growth. This is a slightly different effect, but one that’s no less important: gravitationally bound structures grow over time, as more and more matter falls into them. Gravitation is a runaway force in the Universe in the sense that if you start with a uniform Universe, where everywhere around you has the same density except for one location that’s a slight amount denser than average, that region will progressively swallow up more and more of the surrounding matter over time. The more mass you have in one region, the greater the gravitational force gets, making it easier to attract more and more mass as time goes on.

Now, let’s imagine that you’re a dark matter particle that happens to fall into one of these gravitationally growing regions. You enter this region with a small but positive speed, drawn in by the total amount of mass inside that region. As you fall towards the center of this region, you accelerate based on the amount of mass that’s in there now. But as you fall in, other masses fall in as well — some of which are normal matter and some of which are dark matter — increasing the density and the total mass of where you are.

The evolution of large-scale structure in the Universe, from an early, uniform state to the . [+] clustered Universe we know today. The type and abundance of dark matter would deliver a vastly different Universe if we altered what our Universe possesses. Note the fact that small-scale structure appears early on in all cases, while structure on larger scales does not arise until much later, but that structures become denser and clumpier as time goes on in all instances.


You reach the periapsis of your orbit (the closest approach to the center-of-mass of the structure you’re inside), and now you begin the long journey back out. But the amount of mass that’s now tugging back on you, that you need to overcome to get back out, has grown over time. It’s as though you fell into a solar system with the mass of our Sun, but as you go to leave, you find that you’re trying to escape from a solar system with a mass that’s a few percentage points more massive than our Sun. Which means, overall, that if you were moving slowly enough when you first fell in, you won’t be able to get back out, and you’ll remain gravitationally bound.

In reality, these two effects are both at play, and while either one can lead to dark matter becoming a part of the gravitationally bound large-scale structures in the Universe, their combined effect is even more significant. When you simulate how structure in the Universe forms with both of these effects included, you find that not only does dark matter make up the majority of the mass in these bound structures that arise, but that even if you simulated a Universe that only had dark matter — with no normal matter at all — it would still form a vast cosmic web of structure.

This snippet from a structure-formation simulation, with the expansion of the Universe scaled out, . [+] represents billions of years of gravitational growth in a dark matter-rich Universe. Note that filaments and rich clusters, which form at the intersection of filaments, arise primarily due to dark matter normal matter plays only a minor role.

Ralf Kähler and Tom Abel (KIPAC)/Oliver Hahn

If the Universe were as Einstein originally envisioned it — static and unchanging with time — then dark matter particles wouldn’t become gravitationally bound at all. Any structure that a dark matter particle fell into would, a specific time later, see that dark matter particle escape once again: a situation that would apply equally to planets, solar system, galaxies, and even galaxy clusters.

But because the Universe expands, reducing the kinetic energy of particles traveling through it, and because structures also gravitationally grow over time, meaning that a particle that falls in has a tougher time getting back out again, dark matter particles wind up gravitationally bound inside these structures. Even though they don’t collide, exchange momentum, or otherwise dissipate energy, they still contribute in a meaningful way to the large-scale structure of the Universe. While only the normal matter collapses down to form ultra-dense structures like stars and planets, the dark matter remains in large, diffuse halos and filaments. When it comes to the large-scale structure of the Universe, dark matter’s presence has a clear effect that we simply can’t ignore.

Light Pollution Harms the Environment

In general, the most common action is that light pollution alters and interferes with the timing of necessary biological activities. For approximately half of all life, those crepuscular and nocturnal species that begin their daily activities at sundown, our artificial lights at night seriously constrain their lives, exposing them to predators and reducing the time they have to find food, shelter, or mates and reproduce.

To assume that other living organisms on this planet are just going to "adapt" to our newly created lighting schedules for commericial convenience is apathetically ignorant and insane. Unfortunately, it is far, far easier to setup a badly installed light outside than it is to understand the negative effects it casts down-light from it. For example, U.S. roadways contribute a huge amount of waste light. All of that bad lighting could be redone by replacing the up-pointing 300W halogen bulbs with more efficient LED lights and by pointing the LEDs downwards. Overtime, this would cost far less for the taxpayers without causing a single change in the quality of information delivered to the traveler or to compromise their safety. Such lighting seems especially wasteful as headlights on cars are more than sufficient to light up signs, so the lights are not truly needed at all now.

However, bad lighting does not stop just at the roads. Tiffany Saleh wrote a good article on the "Effects of Artificial Lighting on Wildlife" explaining this in the's web site.

This page provides an organizational overhead to some of the impacts that light pollution has on different species which have lived on this planet far longer than us "john-come-lately" humans. The dark blue menu column on the left will also help you navigate these pages as they grow.

In the span of a mere one hundred years, our creation of a never-occurring night is having some real effects on the animals that were here before us. For the same melatonin suppression problems we have with lights at night, creates similar problems in animals. Melatonin is the chronobiotic hormonal regulator of neoplastic cell growth, meaning that it is just the hormonal signal of our biological clock , is used for such functions in mammals worldwide. Biologists describe it as being the most evolutionary conservative hormone that we know of, meaning that it is one of the oldest hormones known across the tree of life that basically signals to genes and organs whether it is daytime or not. Hence light pollution affects animals as well. A mere glance at the articles in the Light pollution vs. Human Health pages easily confirms this fact as melatonin testing is done over and over on rat species. In fact, it is found in almost all organisms.

But melatonin is more than just some ancient hormone buried deep within us and the animals that is being impacted. Night tells so many animals when to eat, when to sleep, when to hunt, when to migrate or even when to reproduce, it is estimated that half of all life on earth start their daily activities at sundown. Here is a brief, incomplete accounting of how light pollution harms those living outside our materialistic world.

Our Vanishing Night -- YouTube video by astrogirlwest

What happens when all the dark places are gone?
Light pollution: its real, destructive consequences are seldom recognized, but it is a problem with easy solutions that make economic sense. All living creatures rely on the Earth's regular rhythm of day and night to regulate internal cycles. Many use the protection of darkness to safely forage and mate. We exist in a balance with our environment, a delicate balance that we are shifting. In the process we are also losing our connection to the night sky and the universe beyond.

FLASH: A new study reports that lights on at night can worsen smog conditions for a city! Sunlight breaks down the nitrate radical NO3, so its levels build up during the night. As it does so, it neutralizes some of the other nitrogen oxides (NOx) that contribute to smog. But it is not just sunlight that can break down NO3 any light can do this, especially those city lights that are left on all night long. Streetlights are often immediately next to the sources of the exhaust creating smog and are measured to be about 25 times stronger than the light of a full Moon. This combined effect reduces the natural cleaner NO3's levels down by 7%, which then increases the smog components by a non-negligible 5%.

Now add to that the practice of additional outdoor lights on for the holiday season .

Click below to additional pages covering effects of light pollution on plants and animals in:

The old Lakota was wise. He knew that man's heart away from nature becomes hard he knew that lack of respect for growing, living things soon led to lack of respect for humans too.

-- Chief Luther Standing Bear

Links to Other Sites

The Florida Fish and Wildlife Conservation Commission has their own pages about light pollution effects on various biological organisms. Check them out as they are in the field, obseving these effects directly!

The out in California has their own Ecological Light Pollution page covering additional articles and reports on the effects of lights at night and the environment.

light pollution harms the environment harms animals environmental problem Florida Palm Beach County Broward County Miami Dade County light pollution in the deserst Las Vegas light pollution

W ayne h u

Key Concepts

  • Position of peaks mainly sensitive to curvature
  • Shapes fixed by the physical density of matter and baryons
  • Missing or " dark energy " plays a small roll in the position of peaks

As advertised, the position of that first peak in the power spectrum of the anisotropies, and indeed all of the peaks, depend sensitively on the spatial curvature of the universe. As the curvature of the universe decreases (and in fact becomes negative in the yellow curve below)

the peaks move to smaller angles (higher multipole l) while preserving their shape. Cosmologists fraction of the critical density in matter &Omega m so that as 1-&Omega m increases from zero, the universe becomes increasingly negatively curved if there is no other forms of "missing energy" that we've missed in our accounting.

In the blue curve, we assume that there is indeed a form of "missing" or dark energy in a cosmological constant that makes the universe flat despite a sub-critical density of matter. We see that the positions mainly care about the curvature of the universe but there is a small shift to larger angular scales (lower multipoles) through the introduction of a cosmological constant. This is because the cosmological constant produces a small change in the distance light can travel since recombination , a fact that is related to its well known effect on the age of the universe.

Note that in the above figure we assume that the physical density of the matter (&Omega m h 2 ) is fixed. This is different from saying its fraction of the critical density (&Omega m ) is fixed since the Hubble constant (h), or expansion rate today, enters into the definition of the critical density. Given our current uncertainties about the physical density of the matter, the distance that sound can travel by recombination is currently uncertain, a fact related to the matter's effect on the age of the universe at recombination. Likewise we have assumed that the physical density of the baryons (&Omega b h 2 ) is fixed. Baryons lower the speed of sound in the medium and hence also affect the distance sound can travel. Luckily both of these quantities dramatically change the shape of the peaks as we shall see. They will be measured once the higher peaks are detected and will no longer confuse the measurement of the curvature. As it turns out, the sound horizon is not a standard ruler but rather a standardizeable ruler .

How does light affect the universe? - Astronomy

A lot about the world has been learned during the past few centuries, but some mysteries still remain. Apparently, Nature has not revealed to us all her secrets but scientists are hard at work trying to decipher them.

On August 28, 1999, National Public Radio's Science Friday program presented some of the greatest unsolved problems in science. Here is Jupiter Scientific's list.

More detailed information will be provided in the coming months, so come back and find out more.

In Astronomy: The Mystery of Dark Matter

What Is Dark Matter?
Astronomers have discovered that there is an amazing halo of mysterious invisible material that engulfs galaxies and clusters of galaxies. Astronomers have no idea what it is, and it composes about 95% of the mass of the Universe.
If It's Invisible, Then How Does One Sense Its Existence?
Astronomers have detected dark matter indirectly through its gravitational pull. For example, dark matter causes the stars in the outer regions of a galaxy to orbit faster than they would if there was only ordinary matter present.
What Are Some of the Speculations as to Its Composition?
If neutrinos have mass, they might be a component of dark matter. Black holes and undiscovered, exotic elementary particles are other possibilities.
Why Is Dark Matter Important?
Dark matter played a crucial role in galaxy formation during the evolution of the cosmos. It also determines the ultimate fate of the Universe.

Click here for additional information about dark matter.
To read a transcipt of a radio broadcast on dark matter, click here.

In Gravity: The Construction of a Consistent Quantum Theory of Gravity

What Is the Difficulty?
The theory of gravity as formulated by Einstein is incompatible with the rules of quantum mechanics. Theorists encounter serious difficulties when trying to construct a quantum version of gravity.
How Would Our Understanding of Gravity Be Affected?
A quantum gravity theory would lead to few noticeable effects in the macroscopic world. At distances much, much smaller than an atom, however, Einstein's gravity theory would be significantly changed.
Why Is This Problem Important?
Quantum mechanics and gravity are two great pillars of science. The marriage of these two principles would create a new fundamental understanding of Nature. There could also be implications for black holes.

For a more detailed discussion, click here.

In Particle Physics: The Mechanism That Makes Fundamental Mass

What Is the Problem?
The masses of the electron, proton and neutron are generated through what-is-called "electroweak breaking," but particle physicists do not know how this breaking mechanism works.
Does "Electroweak Breaking" Affect the Macroscopic World?
Since all material is made of atoms and since atoms are made of electrons, protons and neutrons, the "breaking" produces the mass of everything.
When Are Scientists Likely to Solve This Problem?
The Large Hadronic Collider, which is being built near Geneva, Switzerland, should be completed around the year 2005. By the end of the next decade, physicists probably will know the answer.

Click here for more information. See also the Jupiter Scientific report on the Possible Discovery of the "God Particle".

In Theoretical High-Energy Physics: The Unification of the Basic Forces

In Cosmology: The Creation of the Universe

In Biology: How the Basic Processes of Life Are Carried Out by DNA and Proteins

In Neuroscience: Free Will

Other Important Scientific Problems:

In Astrophysics: The Source of Gamma Ray Bursts

In Theoretical Cosmology and Particle Physics: The Cosmological Constant Problem (A caller on the August 28 Science Friday broadcast asked a question about this important issue)

Update: Recent observations indicate that the Universe is accelerating and therefore a non-zero cosmological is likely. This increases the importance of the cosmological constant problem. However, theorists have invented a new solution call quintessence.

In Particle Physics and Astrophysics: The Solar Neutrino Problem

Update: This problem is most likely solved. There is strong evidence that neutrinos have mass and that electron neutrinos emitted in the core of the Sun transform into other neutrinos via oscillations on their way to the Earth.

In Solid State Physics: The Mechanism Behind High-Temperature Superconductors

In Biology: Protein Folding

In Neuroscience: Consciousness (Floyd Bluhm, the editor of Science magazine, raised this important issue in the August 28 Science Friday broadcast)

In Paleontology: How Present-Day Microbiological Information Can Be Used to Reconstruct "The Ancient Tree of Life"

In Geology: The Dynamics of the Inner Earth

In Geology: Earthquake Predicting

In Chemistry: How Microscopic Atomic Forces Produce Various Macroscopic Behaviors

In Chemistry: The Fabrication and Manipulation of Carbon-Based Structures (Fullerenes)

If you have a suggestion for this list of great unsolved problems in science, please e-mail it to [email protected]

This report was prepared by the staff of Jupiter Scientific, an organization devoted to the promotion of science through books, the internet and other means of communication.

This web page may NOT be copied onto other web sites, but other sites may link to this page.

Q: Why does the entropy of the universe always increase, and what is the heat death of the universe?

Physicist: The increase of entropy is just how a scientist talks about the fact that the universe tends to do the most likely thing. For example, if you throw a bucket of dice you’ll find that about a sixth of them will be 1, about a sixth will be 2, and so on. This has the most ways of happening, so it’s the most likely outcome, and for the same reason it’s the outcome with the highest entropy.

High entropy. Arrangements of lots of dice tend, over time, to end up like this.

In contrast, you wouldn’t expect all of the dice to be 4 at the same time, or otherwise assume one particular pattern. That would be a very unlikely and low entropy outcome.

Audrey Hepburn is one of the lower entropy states you’ll find. Or rather, will never find, because it’s so unlikely. You have to sit back and squint a little to see it.

“Entropy” is just a mathematical tool for extending the idea down to atomic interactions, where we don’t have a nice idea like “dice” to work with.

One of the things that increasing entropy does is to spread out heat as much as possible. If you have a hot object next to a cold object, then the heat will spread so that the cooler object heats up, and the hotter object cools down, until the two are at the same temperature. The idea (the math) behind that is the same as the idea behind mixing fluids or sands together. There are more ways for things to be mixed than sorted.

The same thing happens on a much larger scale. The Sun, and every other star, is radiating heat into the universe. But they can’t do it forever. Eventually the heat will have spread out so much that there won’t be warmer objects and cooler objects. Everything will be the same temperature. The same, very cold, temperature. The vast majority of the universe is already screaming cold, so the heat death of the universe is just about burning what fuel there is and mixing the heat so created into the ever-expansive, cold, and unyielding cosmos. Both the burning of fuel (mostly through fusion in stars) and the distribution of heat are processes which increase entropy.

The cold and unyielding cosmos. What’s the stupid point of anything?

Once everything is the same temperature, the universe attains a “steady state”. With no energy differences, there’s no reason for anything to change what it’s doing (all forces can be expressed as an energy imbalance or “potential gradient“). Heat death is the point at which the universe has finally settled down completely (or almost completely), and nothing interesting ever happens again.

Which is depressing, but it is a fantastically long time away. There are a hell of a lot of other bad things that’ll probably happen first.

The eminent philosophers Flanders and Swann have a more up beat take on the heat death of the universe:

“Heat is work, and work’s a curse,

and all the heat in the universe,

is gonna cool down. ‘Cause it can’t increase,

then there’ll be no more work, and there’ll be perfect peace.

Bringing the Universe to Classrooms and Homes around the World!

Sir Isaac Newton didn't use his telescope to find any new things in the universe but he did use it to radically transform how we view the world we live in and the universe as a whole.

Sir Isaac Newton is often considered as the greatest Astronomer and Mathematician to ever live. There is a lot of validity to this claim. This article looks at his famous reflector telescope and describes some of his discoveries.

A reflector telescope is one that uses a mirror rather than lenses to bend light and magnify images. Reflector telescopes, because they are easier to make and can be made in sizes much larger than refractors, are an invention that changed astronomy and our understanding of the universe. The largest refractor telescope in the world is forty inches in diameter and reflector telescopes dwarf this in comparison. There are currently several reflector type scopes that are over four hundred inches in diameter.

Sir Isaac Newton surrounded by symbols of some of his greatest findings.
Illustration by Jean-Leon Huens, National Geographic Stock.

Why a reflector is better than a refractor

If you are familiar with a prism or a rainbow you can understand why reflectors are superior to refractors. When light passes through the glass the different bands (or colors) pass through at different angles and this causes aberrations or problems in the images. This is called chromatic aberration and it gives us distorted views of what we see through a lens. In the time of Newton glass making and lens making was very primitive and the problems of chromatic aberration were not yet overcome. Today we can make lenses that have almost no chromatic aberration but we can’t make them very large. When a lens gets to be really large it gets very heavy and its own weight will distort the lens and ruin the image.

Newton’s telescope solved these problems. A mirror doesn’t pass light through it. It simply bounces all the light off the surface. There is no chromatic aberration at all. And because you only need to bounce light off the surface you can place the whole mirror on a supporting structure or base which takes a lot of the weight off the mirror. This way you can build much larger mirrors without any distortion.

It is commonly thought that Newton invented the first reflector telescope but it isn’t true. Credit for making the first reflector goes to an Italian Monk, Physicist, and Astronomer named Niccolo Zucchi. He published a book on Optics in the 1650s and it is this book that inspired Sir Isaac Newton to build his own telescope. Zucchi created his first reflector around 1616 while Newton completed his first (and famous) telescope in 1670. But while Zucchi did make some new discoveries with his telescope it didn’t work well and was difficult to make and to use. It was Newton’s telescope that worked really well and that brought the art and science of reflectors into the world of science.

The real genius of Newton’s Telescope

All of that stuff is remarkable but there is something much more important in Newton’s Astronomy and in his telescope. He didn’t after all, discover moons around Jupiter like Galileo did, or plot the return of a comet-like Halley did. But what he did do was tie in Mathematics, Astronomy, and our understanding of the universe using his telescope and his theory of universal gravitation. He proved mathematically that gravitation was a two-way operation and that while the earth pulled on a falling apple so the apple too pulled on the earth. This was clearly seen, calculated Psychology Articles, and confirmed in the motions of heavenly bodies which was refined and made possible by the new science of reflector telescopes which we can credit to Newton.

Sir Isaac and his telescope carried on with the work of Copernicus and Galileo by furthering our understanding of the universe we live in and helping us to realize there are laws that govern the whole of the universe. And this rule holds true for falling apples and for planets revolving around stars.

How Fast Does Light Travel in Water vs. Air? Refraction Experiment

How fast does light travel, and does it travel faster in water or air? The fastest thing in the whole universe is the speed of light in a vacuum (like outer space!), clocking in at a great 2.99 x 10 8 m/s. Light travels in waves, and we call this traveling propagation. Propagation of waves has both a speed and a direction, called the velocity. The velocity of light changes depends on the material it travels through.

Light waves can be changed in a few different ways. Reflection is when the waves bounce off a surface and change direction, like when they hit a mirror or pool of water. Diffraction spreads out light waves an example of this is water vapor in the air diffracting light from the sun to create a rainbow. The third type of light behavior is refraction. Refraction is where light waves pass through a material (what scientists call a medium) and change direction. Have you ever stuck your arm beneath the surface of the water in a fountain or swimming pool, and wondered why it looks like it has a sharp bend in it right at the surface? This is because of refraction!

In this project, you will use a laser to measure refraction through different media. Laser is an acronym for &ldquoLight Amplification by Stimulated Emission of Radiation,&rdquo which in simple terms means you are firing beams of light in a straight line.


How does light refract differently when traveling through different media?


  • Sheets of paper
  • Pencil
  • Colored marker
  • Ruler
  • Protractor
  • Calculator
  • Rectangular transparent material at least ¼&rdquo thick. Some examples include:
    • Glass
    • Plexiglass
    • Plastic
    • Gelatin
    • Glass dish filled with water
    • Clear plastic dish filled with water


    1. Fold a clean sheet of paper in half.
    2. Place one of the test materials on the folded sheet of paper so the centerline of the object is on the fold.
    3. Trace the outline of the object onto the paper with the pencil.
    4. Use a colored marker to make a small dot on the edge of the sheet. This is where you will aim the laser. This dot should be on the same side as the fold, at least 1.5 inches from the fold. Why should the place where the laser will be aimed be marked?
    5. Lay the laser down on the table or countertop and adjust the beam so it enters the page at the colored dot you made and hits the object at the centerline fold.

    1. Turn the lights off if it makes it easier to see the laser beam.
    2. Mark the laser beam path in and out of the object with a few dots using the pencil.
    3. Use the protractor to measure the angle of incidence and angle of refraction. Record the data and be sure to include any observations. The angle of incidence (&theta1) is the angular distance from a reference (in this case the centerline fold) at which the laser beam approaches and hits the object. In this case, the medium is air. The angle of refraction (&theta2) is the angular distance from a reference (in this case, the centerline fold) that light travels through the new medium:


    Light will have the fastest velocity when it travels through the air. Light will have the slowest velocity when it travels through gelatin.

    Light slows down when passing through different transparent materials. The more it slows down, the more it bends when it hits a medium made of that material. Snell&rsquos Law of Refraction shows the relationship between incidence and refraction angles and the phase velocities of the materials involved. For this experiment, your laser beam traveled through an air phase before hitting the phase of whatever solid you chose. Snell&rsquos law states that the ratio of the sine of the incidence to the refraction angles, &theta, is equal to the ratio of the phase velocities, v.

    Another variation on Snell&rsquos law includes the index of refraction, n. The previously stated Snell&rsquos law is equal to the reciprocal of the ratio of the indices of refraction.

    The index of refraction is a dimensionless number, or a number without any units. Dimensionless numbers are used to be able to compare two different objects on the same parameters. The index of refraction describe how light travels through a medium.

    Where c is the speed of light in a vacuum (2.99 x 10 8 m/s) and v is the speed of light in the medium you are measuring in m/s.

    Going Further

    Try adding salt or sugar to the water in the container and perform the experiment again. What happens? Is the velocity different when you dissolve solids in the liquid? You can also try measuring other see-through liquids like clear soda or liquid soap. You can also try using different shaped objects like prisms to see how light is refracted differently.

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    Warning is hereby given that not all Project Ideas are appropriate for all individuals or in all circumstances. Implementation of any Science Project Idea should be undertaken only in appropriate settings and with appropriate parental or other supervision. Reading and following the safety precautions of all materials used in a project is the sole responsibility of each individual. For further information, consult your state's handbook of Science Safety.

    Short Answers To Big Questions: What Is Dark Matter?

    Short Answers To Big Questions: What Is Dark Matter?

    "Dark matter is a very big mystery," Barkana says. "We know it's most of the matter in the universe, but we have no idea about its properties."

    Scientists only know about dark matter because they have observed the effect of its gravity. "Other than gravity, we haven't had any clues," he says.

    He began to think about dark matter in the context of the first stars, and did some calculations. His work suggests the hydrogen gas could be cooled by interactions with dark matter particles that are relatively light, as opposed to the heavier mass people have been theorizing.

    "The idea that a detectable radio signal from the cosmic dawn can be connected to the particle properties of dark matter suggests a potentially revolutionary angle for exploring fundamental physics," Lincoln Greenhill of Harvard University wrote in an opinion article for Nature about the new work.

    Everyone agrees that another group working independently needs to confirm the existence of this radio signal from the early universe. "It's very important that this whole result we've got definitely needs to be confirmed, absolutely," says Rogers.

    But if the temperature discrepancy holds up, then the argument over what explains it can really begin in earnest. "It's a really, really interesting result, and a very exciting one as well," says Katie Mack, an astrophysicist at North Carolina State University. "This was not something that was predicted by any of the usual astrophysical models."

    With the exception of the afterglow of the Big Bang itself — the cosmic microwave background — this observation marks the farthest back in time that scientists ever have been able to investigate, says Mack.

    "It's the earliest detection of any kind of astrophysics, ever," she says. "This is a signature of the very first stars in the universe, and the very first black holes in the universe. This is way earlier than anything else."

    The idea that dark matter might play a role makes this even more intriguing. "If that's the case, then we've detected the first non-gravitational interaction between dark matter and anything," says Mack, who says this may turn out to be the first evidence that "dark matter does anything at all other than sit there and gravitate."