# Galaxies receeding at and above speed of light

## Main Question or Discussion Point

I am not a physicist but I am an engineer who uses your science. Cosmology is fascinating and what I know about it is not much and much of which I've learned from programs such as NOVA and from a 1960 college course in Modern Physics. My question is from some information I read or heard several years ago and that is-- are some galaxies receeding from each other at or above speed of light and if so how is it verified or postulated?

marcus
Gold Member
Dearly Missed
Recession is not ordinary motion, so there is nothing remarkable about distances increasing at a rate greater than c, any more than about distances increasing at a rate less than c.

In fact it happens to be typical. If you pick two galaxies at random within our observable range, more likely than not the distance between is increasing at a rate > c. And typically both will be approximately at rest relative to background.

How can you tell? The same way you tell any recession rate, whether it is greater than c or less than c. You look at the redshift. If the redshift is greater than 1.4 then the distance to the galaxy was increasing > c at the time it emitted the light.

If the redshift is greater than 1.7 then the present distance to the galaxy is currently increasing > c, today as we receive the light.

We see galaxies out to redshift 6 or 7 (in some cases more) so of course there are plenty in our visible range with redshift > 1.7 and even more with it > 1.4.

DrChinese
Gold Member
My question is from some information I read or heard several years ago and that is-- are some galaxies receeding from each other at or above speed of light and if so how is it verified or postulated?
You may benefit from this, the authors have written extensively on the subject:

As Marcus says, we do see the light from galaxies which have always been receding >c from us. How is that possible? Lineweaver and Davis explain that the light moves from one area of space to another. As it heads our way, eventually it gets to an area of space that is not expanding as fast relative to us as where it originated. Later, it moves to an area of space which is close enough to us (even though far away) that the space is expanding away from us at less than c. At that point, the light starts to approach us. Slowly it begins its approach and gains speed. Eventually it approaches us at c when it comes to our neck of the woods - perhaps the last billion years of its journey.

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Is this to say that light entering a rapid space expansion 'zone' may travel at a less than c speed?

marcus
Gold Member
Dearly Missed
Is this to say that light entering a rapid space expansion 'zone' may travel at a less than c speed?
No. In conventional cosmology light always travels at the one fixed speed.

You can get an idea of what that means if you spend a while watching the balloon model.
Google "wright balloon model" and watch it.
The wrigglers are photons and they always travel at a fixed speed, like one millimeter per second, across the face of the balloon.

The white whirling spots are galaxies. They do not travel, they stay always at the same latitude longitude on the balloon surface (though of course they get farther apart as balloon distances increase.) You can watch a photon travel from one galaxy towards another.

Imagine you are in Galaxy A and that Galaxy B sends a photon in your direction then, even though the photon is traveling towards you at the usual constant speed of 1 mm per second, for a while the photon may be getting farther away.

(simply because distances are increasing, without the galaxies moving.)

Watching the balloon model animation is a good way to get into your head about the dynamic changing geometry of space.

You can see this happen if you watch alertly: a photon can start out traveling towards you at the usual speed---and at first it will be getting farther away---and it continues getting farther away for a while (although always traveling towards you)---and then the expansion rate which is a percentage rate, slows.

So the balloon distances are increasing now by a smaller percentage per minute than they were before. You can see this happen.

And you will see the photon is now beginning to get closer to what it is traveling toward, instead of always getting farther away from its destination, as was happening before.

The speed is constant, but the geometry changes. Watching helps you understand.

http://www.astro.ucla.edu/~wright/Balloon2.html

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So if I understand the ballon analogy and the recession > c, as the photons travel from one galaxy to our own, the photons travel in a curved path (surface of the balloon). Before reaching a point ( lets's say the halfway point if the two galaxies were on the same latitude) on the surface of the balloon, the photons appear to recede from us but past that point the photons approach us. The photons velocity would be tangential to the curved path and that velocity would be = to c.

marcus
Gold Member
Dearly Missed
So if I understand the ballon analogy...the photons travel in a curved path ...
What I see happening, the essential thing the model shows, does not depend on the surface being curved. It would happen on a flat infinite sheet as well.

Lineweaver and Davis have a pedagogical article in the SciAm where they discuss expansion basics, and at one point they describe it with a flat picture. The link is in my signature, if you havent already read the article perhaps you should.

It is the princeton.edu link. Look on page 42 in a box with two graphic illustrations. It is a short article beginning around page 37. They use the balloon model as an illustration at the beginning but they also discuss the case where there is no overall curvature. I'm talking about 5 pages in from the beginning of the article.

==quote==
CAN WE SEE GALAXIES RECEDING FASTER THAN LIGHT?

WRONG: Of course not. Light from those galaxies never
reaches us.

A galaxy farther than the Hubble
distance (sphere) recedes from
us faster than light. It emits a
photon (yellow squiggle). As
space expands, the photon is
dragged away like someone
trying to swim against the
current. The photon never
reaches us.

RIGHT: Sure we can, because the expansion rate changes
over time.

The photon initially is unable
to approach us. But the Hubble
distance is not constant; it
is increasing and can grow to
encompass the photon.
Once
that happens, the photon
approaches us and eventually
reaches us.

==endquote==

To understand this properly you probably need to know what the Hubble rate is. It varies with time. Call it H(t) and then the Hubble distance is defined to be c/H(t).
And since H(t) is decreasing, the distance (which acts like a reciprocal) is increasing.

I don't know if you are going to get it this time round. But if you want to try you might give the Lineweaver article a careful reading and then ask some more here.

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Chronos
Gold Member
We are surrounded by the surface of last scattering [CMB] , which is about 13.3 billion light years distant with a redshift around z~1100 [at present] in every direction. Everything we presently observe, as well as everything possible to obserse [in the EM spectrum], resides in the foreground of the surface of last scattering.

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Chronos
Gold Member
Allow me to clarify: it is possible we reside on an infinitely tiny 'bubble' of existence on this surface of last scattering. Current data does not affirm or refute this hypothesis.

RUTA
We are surrounded by the surface of last scattering [CMB] , which is about 13.3 billion light years distant with a redshift around z~1100 [at present] in every direction. Everything we presently observe, as well as everything possible to obserse [in the EM spectrum], resides in the foreground of the surface of last scattering.
Here is another quote from the SciAm article:

HOW LARGE IS THE OBSERVABLE UNIVERSE?

Wrong: The universe is 14 billion yrs old, so the radius of the observable part is 14 billion light-years.

Right: Because space is expanding, the observable part of the univer has a radius of more than 14 billion light-years.
*****************************

In the flat, matter-dominated model the observable part is 3c times the age, i.e., 3 times 14 billion light-years = 42 billion light-years (which is about what you get for z = 1100). See, "Can galaxies exist within our particle horizon with Hubble recessional velocities greater than c?" W.M. Stuckey, Am. J. Phys. v60, #2, Feb 1992, pp 142-146.

marcus
Gold Member
Dearly Missed
Here is another quote from the SciAm article:

HOW LARGE IS THE OBSERVABLE UNIVERSE?

Wrong: The universe is 14 billion yrs old, so the radius of the observable part is 14 billion light-years.

Right: Because space is expanding, the observable part of the univer has a radius of more than 14 billion light-years.
*****************************

In the flat, matter-dominated model the observable part is 3c times the age, i.e., 3 times 14 billion light-years = 42 billion light-years (which is about what you get for z = 1100). See, "Can galaxies exist within our particle horizon with Hubble recessional velocities greater than c?" W.M. Stuckey, Am. J. Phys. v60, #2, Feb 1992, pp 142-146.
Thanks RUTA,

it helps when other people chime in on this instead of just ignoring this mis-use of words.

A naive person can easily get the idea that when current distance to source is quoted in lightyears this refers to light travel time.

Most of us realize that when cosmologists use the word they are talking about actual distance, not time, and that there is a fixed conversion between lightyear and meters.
Distances-to-source (whether expressed in lightyears or meters or parsecs or centimeters ) are determined differently from light travel times.

A light year is 9.46 x 1015 meters.

A lot of people (including Ned Wright on occasion!) use that approximate factor of 3 that you mentioned. The actual ratio relating travel time and distance to source varies and depends on the history of expansion. In practice it usually works out to be a bit over three.

So you get numbers like the CMB light travel time is 13.7 billion years and the distance to source is 46 billion lightyears.

Or with some new parameters the travel time is 13.3 billion years and the distance is 45 billion lightyears.

(As we both know the light has not actually traveled all that way, it travels some distance along the way and then that distance expands. So the light is being helped by expansion. It is not really traveling faster than light. And the expansion rate is not fixed, so there is no simple relation between travel time and current distance-to-source.)

Anyway thanks and please continue correcting that misuse of terminology whenever you spot it! We've got a responsibility to newcomers who can get confused on this point!

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Chronos
Gold Member
My point is based on:

http://curious.astro.cornell.edu/question.php?number=72 [Broken]

“ ... You might say that the most distant object visible from Earth is the Cosmic Microwave Background, the remaining heat from the Big Bang which is visible all around us. The Background is found at a distance of about 15 billion light-years from us in all directions. ...

http://curious.astro.cornell.edu/que...php?number=151 [Broken]

“ ... From the current rate of expansion of the Universe, astronomers infer that the age of the observable Universe is about 15 billion years. In other words, if we assume that the Universe has been expanding at a constant rate since the Big Bang, then the rate of expansion tells us how far back in time the expansion started, which we take to be the beginning of the Universe. If the Universe is 15 billion years old, then light has had 15 billion years to propagate, and so the statements "15 billion years old" and "fifteen billion light years apart" are completely equivalent. ...”

http://curious.astro.cornell.edu/que...php?number=476 [Broken]

“ ... The universe may be infinite, but we can only see a finite section of it due to the finite speed of light. We can only see those parts from which light has had time to reach us since the beginning of the universe - which means we can (in theory) see a spherical universe with radius of about 15 billion light years. ...”

I agree the observable universe may be older than inferred from light travel time due to accelerated expansion, as noted in http://curious.astro.cornell.edu/que...php?number=151 [Broken]

" ... there is recent evidence that the rate of expansion of the Universe is increasing with time; that is, galaxies are moving away from each other *faster* today than they were in the past. This means that the observable Universe is *more* than 15 billion years old. ..."

I did not perceive it necessary to address this point in this particular discussion.

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sylas
My point is based on:

http://curious.astro.cornell.edu/question.php?number=72 [Broken]

“ ... You might say that the most distant object visible from Earth is the Cosmic Microwave Background, the remaining heat from the Big Bang which is visible all around us. The Background is found at a distance of about 15 billion light-years from us in all directions. ...
A bit out of date.

There are several different notions of distance. The one that makes sense with this number being used is the "light travel time". That is, the light was emitted about 15 billion years ago (proper time). Although 13.7 is more up to date, and it might be even less with new H measures.

[I presume you were using the more up to date numbers. I can get a new age with the 0.27,0.73 values for mass and dark energy density, but I don't know whether they have new values. Do you know?]

On the other hand, the "co-moving radial distance" is about 45 billion light years. That is, the light was emitted from a region of space which is "now" about 45 billion light years away. And, because the universe is expanding, it was at the time of being emitted about only 0.04 billion light years away (40 million light years).

http://curious.astro.cornell.edu/que...php?number=151 [Broken]

“ ... From the current rate of expansion of the Universe, astronomers infer that the age of the observable Universe is about 15 billion years. In other words, if we assume that the Universe has been expanding at a constant rate since the Big Bang, then the rate of expansion tells us how far back in time the expansion started, which we take to be the beginning of the Universe. If the Universe is 15 billion years old, then light has had 15 billion years to propagate, and so the statements "15 billion years old" and "fifteen billion light years apart" are completely equivalent. ...”
The "empty universe" model. It's a nice co-incidence that it happens to give about the same answer for the age of the universe as the current consensus dark energy/dark matter model. As the questions are for teenagers, I think this is okay, but it's worth noting that it is a simplification.

http://curious.astro.cornell.edu/que...php?number=476 [Broken]

“ ... The universe may be infinite, but we can only see a finite section of it due to the finite speed of light. We can only see those parts from which light has had time to reach us since the beginning of the universe - which means we can (in theory) see a spherical universe with radius of about 15 billion light years. ...”
OK... using light travel time as the distance.

I agree the observable universe may be older than inferred from light travel time due to accelerated expansion, as noted in http://curious.astro.cornell.edu/que...php?number=151 [Broken]

" ... there is recent evidence that the rate of expansion of the Universe is increasing with time; that is, galaxies are moving away from each other *faster* today than they were in the past. This means that the observable Universe is *more* than 15 billion years old. ..."
Urk. This doesn't follow. It depends on a number of parameters. [Acceleration DOES mean an older universe, but by comparison with a flat universe at critical density; not necessarily by comparison with the constant expansion empty universe model.]

If it wasn't for acceleration and dark energy, then the next best model would be lots and lots of dark matter and critical density.

Above, the questions spoke of a "constant rate of expansion". Under that assumption, and using H=71 km/s/Mpc, the background radiation (redshift about z=1100) has traveled for 13.7 billion years. If there was no acceleration, then from flatness we would actually infer an age of about 9.2 billion years. With mass at 0.27 of critical, and a flat universe (hence dark energy at 0.73) we would have an age of 13.7 again.

This is a rather curious co-incidence. I haven't seen much discussion of it, but a couple of cosmologists have noted this. It's strange that the age of the universe so closely matches the age we'd infer from an incorrect empty universe model. There's no reason to expect that, and (in current cosmological models) it is something that only about now in proper time. As time passes, and in the past, this co-incidence does not apply.

In any case dark energy does not mean older than 15 years. It means the age only just manages to get up to the 15 you infer from the obviously incorrect empty universe model.

Cheers -- sylas

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Chronos
Gold Member
For discussion of superluminal recession velocities, see
http://arxiv.org/abs/astro-ph/0310808
Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the Universe
Authors: Tamara M. Davis, Charles H. Lineweaver

" ... We show that we can observe galaxies that have, and always have had, recession velocities greater than the speed of light. We explain why this does not violate special relativity and we link these concepts to observational tests. ..."

I believe this is the paper the SciAm article Marcus referenced is based upon.

Chronos
Gold Member
The observable universe is the issue at hand, IMO. It consists of photons we currently see. The rest is model dependent. I only insist the model is debatable, not the photons. I do not perceive how photons we have not yet observed are relevant.

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marcus
Gold Member
Dearly Missed
Just to be clear here's a copy of Sylas' post #13 responding to Chronos post #12.
Cornell hosts a site aimed primarily at teenagers called "Curious about astronomy? Ask an astronomer." It has modified how it handles distance since 2002. The quotes Chronos uses are from the earlier period 1999-2002. This explains Sylas' comment "A bit out of date." And his comment further down, that "As the questions are for teenagers, I think this is okay, but it's worth noting..."

==quote of Sylas' post==
Originally Posted by Chronos
My point is based on:

http://curious.astro.cornell.edu/question.php?number=72 [Broken]

“ ... You might say that the most distant object visible from Earth is the Cosmic Microwave Background, the remaining heat from the Big Bang which is visible all around us. The Background is found at a distance of about 15 billion light-years from us in all directions. ...​
A bit out of date.

There are several different notions of distance. The one that makes sense with this number being used is the "light travel time". That is, the light was emitted about 15 billion years ago (proper time). Although 13.7 is more up to date, and it might be even less with new H measures.

[I presume you were using the more up to date numbers. I can get a new age with the 0.27,0.73 values for mass and dark energy density, but I don't know whether they have new values. Do you know?]

On the other hand, the "co-moving radial distance" is about 45 billion light years. That is, the light was emitted from a region of space which is "now" about 45 billion light years away. And, because the universe is expanding, it was at the time of being emitted about only 0.04 billion light years away (40 million light years).

http://curious.astro.cornell.edu/question.php?number=151 [Broken]

“ ... From the current rate of expansion of the Universe, astronomers infer that the age of the observable Universe is about 15 billion years. In other words, if we assume that the Universe has been expanding at a constant rate since the Big Bang, then the rate of expansion tells us how far back in time the expansion started, which we take to be the beginning of the Universe. If the Universe is 15 billion years old, then light has had 15 billion years to propagate, and so the statements "15 billion years old" and "fifteen billion light years apart" are completely equivalent. ...”​

The "empty universe" model. It's a nice co-incidence that it happens to give about the same answer for the age of the universe as the current consensus dark energy/dark matter model. As the questions are for teenagers, I think this is okay, but it's worth noting that it is a simplification.

http://curious.astro.cornell.edu/question.php?number=476 [Broken]

“ ... The universe may be infinite, but we can only see a finite section of it due to the finite speed of light. We can only see those parts from which light has had time to reach us since the beginning of the universe - which means we can (in theory) see a spherical universe with radius of about 15 billion light years. ...”​
OK... using light travel time as the distance.

I agree the observable universe may be older than inferred from light travel time due to accelerated expansion, as noted in http://curious.astro.cornell.edu/question.php?number=151 [Broken]

" ... there is recent evidence that the rate of expansion of the Universe is increasing with time; that is, galaxies are moving away from each other *faster* today than they were in the past. This means that the observable Universe is *more* than 15 billion years old. ..."​
Urk. This doesn't follow. It depends on a number of parameters. [Acceleration DOES mean an older universe, but by comparison with a flat universe at critical density; not necessarily by comparison with the constant expansion empty universe model.]

If it wasn't for acceleration and dark energy, then the next best model would be lots and lots of dark matter and critical density.

Above, the questions spoke of a "constant rate of expansion". Under that assumption, and using H=71 km/s/Mpc, the background radiation (redshift about z=1100) has traveled for 13.7 billion years. If there was no acceleration, then from flatness we would actually infer an age of about 9.2 billion years. With mass at 0.27 of critical, and a flat universe (hence dark energy at 0.73) we would have an age of 13.7 again.

This is a rather curious co-incidence. I haven't seen much discussion of it, but a couple of cosmologists have noted this. It's strange that the age of the universe so closely matches the age we'd infer from an incorrect empty universe model. There's no reason to expect that, and (in current cosmological models) it is something that only about now in proper time. As time passes, and in the past, this co-incidence does not apply.

In any case dark energy does not mean older than 15 years. It means the age only just manages to get up to the 15 you infer from the obviously incorrect empty universe model.

Cheers -- sylas

==endquote==

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