# Is the speed of expansion of the universe faster than light?

the speed of light is C and the age of universe is 15 billion years so can't we assume that the radius of the universe can't be more than 15 billion light years? also has the speed of light changed since the beginning of the universe?

Related Special and General Relativity News on Phys.org
ghwellsjr
Gold Member
The universe is actually closer to 13.7 billion years old and the radius of the observable part of the universe is about 13.7 billion light years since the speed of light has always been exactly C. But we assume that the universe is also expanding in the opposite direction and, in fact, in all directions and so there is much more to the universe than we will ever be able to see.

And there are parts of the universe that are moving away from us at faster than the speed of light.

And there are parts of the universe that are moving away from us at faster than the speed of light.
faster than speed of light is it possible ? please elaborate and explain it's evidence

DaveC426913
Gold Member
We believe the universe may be as wide as 156 billion light years.

Relativity forbids massive objects reaching or exceeding c. It says nothing about the expansion of space. Space is expanding and galaxies are moving along with it.

Someone please do correct me if I'm wrong, but I think I have a good way to explain...

No object can move through space at the speed of light relative to another object. No star, meteor, or spaceship will ever pass by earth at a speed equal to or greater than c.

But the expansion of space means that the further away something is from you, the greater the growth of the distance between the the object and you (ie. getting increased distance per unit time). It's almost as if huge amounts of empty space are spawning between the objects every second.

So, neither object is actually "moving" at c, but the growth of space between them is greater than 299,792,458 m/s.

We believe the universe may be as wide as 156 billion light years.

Relativity forbids massive objects reaching or exceeding c. It says nothing about the expansion of space. Space is expanding and galaxies are moving along with it.
Does that mean the distance between sun and earth (in a sense) is not the same as it was say, 1 or 10 billion years ago?

though light would still take 8 mins then and now?

however it would be hard to talk of distance then and now.....but I am not a relativity expert

WannabeNewton
bcrowell already wrote up an FAQ on this:

Last edited by a moderator:
DaveC426913
Gold Member
Does that mean the distance between sun and earth (in a sense) is not the same as it was say, 1 or 10 billion years ago?

though light would still take 8 mins then and now?

however it would be hard to talk of distance then and now.....but I am not a relativity expert
No. This expansion is extremely weak. It is so weak even galaxies ahve no trouble holding together. It only has an effect when gravity is virtually zero - out between galaxy clusters.

I glue two pennies (Earth, Sun) edge-to-edge onto a balloon. Then I put tape across the pennies so they are stuck together (mutual gravity).

I do this again (a star in Virgo galaxy and its planet), so I have two clusters of pennies (Milky Way, Virgo).

I inflate the balloon. If the tape is strong enough then the balloon's inflationary force is far too weak to overcome the tape's strength. The clusters themselves stay together. (Earth-Sun do not change distance).

But the two clusters (Virgo, Milky Way), having no "force" connecting them, move apart as the balloon expands.

pervect
Staff Emeritus
the speed of light is C and the age of universe is 15 billion years so can't we assume that the radius of the universe can't be more than 15 billion light years? also has the speed of light changed since the beginning of the universe?
You might take a look at Ned Wright's FAQ. Here's one part that addresses one of your questions.

http://www.astro.ucla.edu/~wright/cosmology_faq.html#DN

If the Universe is only 14 billion years old, how can we see objects that are now 47 billion light years away?

When talking about the distance of a moving object, we mean the spatial separation NOW, with the positions of both objects specified at the current time. In an expanding Universe this distance NOW is larger than the speed of light times the light travel time due to the increase of separations between objects as the Universe expands. This is not due to any change in the units of space and time, but just caused by things being farther apart now than they used to be.
There is a certain amount of convention involved in defining distances and velocities of very distant objects due to the curvature of space-time. One part of the convention hinges on the issue of "now", the notion of "now" in cosmology is logical and self-consistent, but subtly different from the SR notion of "now" of any particular observer.

faster than speed of light is it possible ? please elaborate and explain it's evidence
An object with mass can never overtake a beam of light, but if space itself is expanding, the distance between two objects can expand at faster than the speed of light. This is possible in the theory of general relativity, where it is possible for geodesics to diverge from one another at this rate.

The strongest evidence that this has occurred in reality is the experimental evidence for the theory of the inflationary universe. Inflation is a quantum theory that predicts the universe underwent several quantum fluctuations during its formation. These fluctuations caused space to expand at faster than the speed of light, which explains the uniformity of the observable universe.

Inflationary theory made several predictions about the universe that weren't predicted by any other theories. These predictions have been experimentally verified with striking accuracy by the WMAP satellite and other studies of the cosmic background radiation.

No. This expansion is extremely weak. It is so weak even galaxies ahve no trouble holding together. It only has an effect when gravity is virtually zero - out between galaxy clusters.

I glue two pennies (Earth, Sun) edge-to-edge onto a balloon. Then I put tape across the pennies so they are stuck together (mutual gravity).

I do this again (a star in Virgo galaxy and its planet), so I have two clusters of pennies (Milky Way, Virgo).

I inflate the balloon. If the tape is strong enough then the balloon's inflationary force is far too weak to overcome the tape's strength. The clusters themselves stay together. (Earth-Sun do not change distance).

But the two clusters (Virgo, Milky Way), having no "force" connecting them, move apart as the balloon expands.
What exactly is it that is expanding?? Also, would that mean casualty can be violated because two objects are seperating from each other faster than the speed of light?

Last edited:
WannabeNewton
This is possible in the theory of general relativity, where it is possible for geodesics to diverge from one another at this rate.
Is there a way to see this come out naturally from the equation of geodesic deviation?

pervect
Staff Emeritus
Is there a way to see this come out naturally from the equation of geodesic deviation?
Try Ned Wright's cosmology FAQ again http://www.astro.ucla.edu/~wright/cosmology_faq.html#SS

Why doesn't the Solar System expand if the whole Universe is expanding?
There's also a reference to a paper by Cooperstock http://xxx.lanl.gov/abs/astro-ph/9803097

See section 2

2 Equations of motion in the LIF
In this section we find the equations of motion for a particle in the LIF using the geodesic
deviation equation. We refer the reader to the Appendix for the details of the calculation.
A short and simplified version is that if the solar system is expanding in terms of it's Fermi frame (the LIF frame mentioned in section 2), it's because it's loosing mass. (in particular, the mass element defined by M in the Schwarzschild geodesic, which will also be equal to the Komar mass, for example).

The dominant mass-loss mechanism is the sun radiating it's energy away, an effect mentioned in the paper but one that is not of cosmological origin. There might be some very very very small cosmological effect due to dark matter.

Cosmologically we know that rho, the mass density (measured locally) is going down,but this doesn't imply that the solar system is loosing mass, necessarily. Dark matter flux is one of the ways that it _might_ be loosing mass due to cosmological effects. And of the effects we do know about, solar radiation dominates, though it wouldn't usually be thought of as cosmological.

What exactly is it that is expanding??
Space itself is expanding, or more accurately, geodesics are diverging from one another over time. This has no effect on what is happening inside galaxies or solar systems, however, because in these systems geodesics are converging.

Also, would that mean casualty can be violated because two objects are seperating from each other faster than the speed of light?
No. Casualty violation would only occur if a worldline traveled outside of the light cone. In other words, it would only occur if an object were to overtake a beam of light. Causality violation doesn't occur because the two objects will never interact with one another.

What exactly is it that is expanding?? If galaxies are moving away from us at greater than the speed of light, due to whatever reason, doesnt that mean the light could never reach us? Also, would that mean casualty can be violated because two objects are seperating from each other faster than the speed of light?
What's expanding is the very spacetime itself. Maybe you prefer to call it the fabric of spacetime? Everything goes for the ride while the cosmos expands.

I am no expert wrt GR, but here's my 2 cents based on what I've read myself ...

Imagine a point in space, where a light source was originally emitting. You were then some distance X away. If spacetime did not expand, it would take the initial photons a duration of T=c/X to reach you. However, since spacetime is stretching everywhere at every moment, the photon has more space (and thus time) to traverse than the original distance X, because it expands as the photon travels. If the expanse separating the POE and yourself were large enough, then the expanding spacetime can prevent a photon from ever reaching you. It seems conceivable to me, that a photon could reach you from (say) the birth of a galaxy some 10 billion years ago, but soon fade from view forever. The way I look at it, it's somewhat analogous to a radially inbound photon attaining an escape velocity "but due to the expansion itself". You have to remember, every point in spacetime is expanding, all the time. The greater the vast expanse, the sooner approaching photons will never be able to reach you, and eventually become outbound.

FAIK, this is no violation of the general theory. Light's invariant speed is assumed true in any local area of spacetime no matter what part of the vast expanse one considers, or when. So the issue of a photon no longer being able to be viewed from another distant perspective is a superpositional effect which does not violate GR.

GrayGhost

You have to remember, every point in spacetime is expanding, all the time.
My understanding is that this isn't entirely accurate. The distance between galaxies in a super cluster is not expanding. Anything that is gravitationally bound isn't affected by the expansion, only objects that are gravitationally separated from one another are.

My understanding is that this isn't entirely accurate. The distance between galaxies in a super cluster is not expanding. Anything that is gravitationally bound isn't affected by the expansion, only objects that are gravitationally separated from one another are.
Is it that gravitation is a superpositional effect that keeps galaxies held together as such even in light of spacetime expansion (everywhere all the time), or that "spacetime itself expands less" if large gravitational fields are present in the region? It appears you assume the latter.

GrayGhost

Last edited:
Is it that gravitation is a superpositional effect that keeps galaxies held together as such even in light of spacetime expansion (everywhere all the time), or that "spacetime itself expands less" if large gravitational fields are present in the region? It appears you assume the latter.

GrayGhost
I'm not an expert in the subject by any means, but my understanding is that spacetime doesn't expand at all in areas where things are gravitationally bound.

I assume this because of the nature of geodesics. Two objects that are being separated by the expansion of the universe have diverging geodesics. Two objects that are gravitationally bound to one another have converging geodesics.

You can't have geodesics that simultaneously converge and diverge at the same time. Right?

We believe the universe may be as wide as 156 billion light years.
Who believe's that?
The visible universe is 12 billion light years, thats why we believed the universe was 12 billion years old before, but now we have calculated indirectly that the universe is about 13.7 billion years old.

George Jones
Staff Emeritus
Gold Member
We believe the universe may be as wide as 156 billion light years.
The radius of the observable universe is 13.7 billion light-years. The radius of the observable universe is 47 billion light-years. The size of universe is at least 78 billion light-years. The popular media have arrived at 156 billion light-years by incorrectly doubling 78 billion light-years.

These statements are seemingly contradictory, but, actually, they are all consistent with current cosmological theory and observations. Why? Because space and distance in cosmology are strange things!

If two different, but correct, ways of measuring distance are used to measure the width of a room, then, to the accuracy of the methods, the results will be same. This is not true in cosmology! Because of spacetime curvature, there is no unique correct concept of spatial distance.

Light travels at a certain finite speed, so the image of any object that we see now is formed by light that left the object some number of years ago. The lookback distance of the object is defined to be the same number of light-years. "the radius of the observable universe is 13.7 billion light-years" uses lookback distance.

Another type of distance is proper distance. Here, "proper" is used in the same sense as "property", not in the same sense as "correct". While the definition of lookback distance considers us and an object at two different times, proper distance considers us and an object at the same instant of cosmic time. "the radius of the observable universe is 47 billion light-years" uses proper distance at the time now.

While lookback distance and proper distance are defined differently, they are both correct. Given distance using one definition, it is possible to calculate the distance given by the other definition. For example, a lookback distance of 13.2 billion light-year corresponds to a proper distance of 31.7 billion light-years.

And there other definitions of cosmic distance. It is important to keep in mind which definition is being used.

Now on to the 78 (156) billion light-year figure. Current cosmological theories indicate that although 4-dimensional spacetime is curved, 3-dimensional space is very close to being flat. For the sake of a shorter explanation, let's assume that space is flat.

Even if space is flat, it might be connected in strange ways, like in the video game Asteroids. In Asteroids, if you fly off the top of the screen, you immediately appear in the same place at the bottom of the screen. Similarly for the sides of the screen. Maybe the universe is connected up in the same way.

A few years ago, some physicists looked at cosmological data and conclude that if the universe is connected like an Asteroids screen, then the width of the Asteroids screen is at least 78 billion light-years. Note: 1) this is something quite different then the size of the observable universe; 2) the popular media took this 78 billion light-year width or diameter incorrectly as a radius and doubled it to get 156 billion light-years.

http://arxiv.org/abs/astro-ph/0310233

ghwellsjr
Gold Member
If spacetime did not expand, it would take the initial photons a duration of T=c/X to reach you.
Don't you mean T=X/c?

JDoolin
Gold Member
Because of spacetime curvature, there is no unique correct concept of spatial distance.

But are there well-defined concepts of spatial distance?

Last edited:
What's expanding is the very spacetime itself. Maybe you prefer to call it the fabric of spacetime? Everything goes for the ride while the cosmos expands.

I am no expert wrt GR, but here's my 2 cents based on what I've read myself ...

Imagine a point in space, where a light source was originally emitting. You were then some distance X away. If spacetime did not expand, it would take the initial photons a duration of T=c/X to reach you. However, since spacetime is stretching everywhere at every moment, the photon has more space (and thus time) to traverse than the original distance X, because it expands as the photon travels. If the expanse separating the POE and yourself were large enough, then the expanding spacetime can prevent a photon from ever reaching you. It seems conceivable to me, that a photon could reach you from (say) the birth of a galaxy some 10 billion years ago, but soon fade from view forever. The way I look at it, it's somewhat analogous to a radially inbound photon attaining an escape velocity "but due to the expansion itself". You have to remember, every point in spacetime is expanding, all the time. The greater the vast expanse, the sooner approaching photons will never be able to reach you, and eventually become outbound.

FAIK, this is no violation of the general theory. Light's invariant speed is assumed true in any local area of spacetime no matter what part of the vast expanse one considers, or when. So the issue of a photon no longer being able to be viewed from another distant perspective is a superpositional effect which does not violate GR.

GrayGhost

First off, I guess this brings up the question of how do we determine what is moving away from each other due to a force, or if space is just being created between them??? Second, if we figure out how this "space" (space time fabric I take it, whatever that is, dark matter maybe?) is being created and make a space building machine, could we then just "create space" between a rocketship and earth and move them apart faster than the speed of light? I know I am swimming way above my head, but I am trying to fill it all in as fast as I can. Thanks for taking my questions seriously, and to all taking the time to help! EDIT: One more question...what do all of you take to stop the migraine headaches all this hard thinking brings on? :)

Don't you mean T=X/c?
Indeed sir. Thanx for the correction. Missed that on my proof-read. T=X/c it is !

GrayGhost

I'm not an expert in the subject by any means, but my understanding is that spacetime doesn't expand at all in areas where things are gravitationally bound.

I assume this because of the nature of geodesics. Two objects that are being separated by the expansion of the universe have diverging geodesics. Two objects that are gravitationally bound to one another have converging geodesics.

You can't have geodesics that simultaneously converge and diverge at the same time. Right?
One might imagine a geodesic that spans from one galaxy to a distant other galaxy. Consider that geodesic locally near either galaxy, while ignoring it everywhere inbetween, and the 2 segments of said geodesic should diverge under continued cosmic expansion (I would figure). Yet, it's still one geodesic in grandier. At any local point in spacetime anywhere, it should appear to converge.

I'd assume your response is what the community would consider to be appropriate. However, let's assume there is a medium of spacetime, and then there are the curvatures thereof. The medium expands under expansion, due to the presumed yet unknown dark energy. I was just wondering whether the medium could be expanding even within a gravity field, while the gravity well maintains it's exact configuration (potential and gradient relative to the COG)? IOWs, could the medium stretch even inside the well, while the well maintain's its precise shape? If the extent of mass dictates the well's geometry, and the mass does not change even during expansion, then maybe an expansion of spacetime inside the well (due to dark energy everywhere) would go unbeknownst within galaxies.

GrayGhost