What is the Speed of Universe Expansion and How Does it Affect Space Travel?

• michael879
In summary, the conversation discusses the approximate speed of the expansion of the universe and how it is affected by redshift and acceleration. Based on Hubble's law, the farther an object is, the faster it appears to be moving away. The Hubble Constant is about 71 kps/Mpc and the universe is currently expanding at a rate slower than the speed of light. However, very distant objects can seem to be expanding faster than the speed of light due to the large distances involved. The conversation also mentions that the universe is accelerating in its expansion, a discovery that is still a mystery. While we cannot observe the entire universe, we can estimate its radius based on the speed of light, the age of the universe, and the
michael879
does anyone know an approximate speed for the expansion of the universe? or if not, how red-shifted planets are? (with the age of universe would give speed). And someone told me its accelerating too. did he mean - acceleration? or is it actually speeding up?

According to Hubble's phenomenological law (a posteriori of exp. datas), the speed of something x away from you is v=Hx...where H is the Hubble constant...so the farther you are, the fastest it seems you go away...but I don't know numerical datas.

From this law you see the acceleration is exponential (it's just first order diff. equ)...but there are other cosmological models...

kleinwolf said:
According to Hubble's phenomenological law (a posteriori of exp. datas), the speed of something x away from you is v=Hx...where H is the Hubble constant...so the farther you are, the fastest it seems you go away...but I don't know numerical datas.

From this law you see the acceleration is exponential (it's just first order diff. equ)...but there are other cosmological models...
ok what I am rly trying to ask is, is the universe expanding anywhere near the speed of light? I am talking about radial expansion, not surface expansion.

That is the simple extrapolation of the Hubble's law - at the radius of the Hubble sphere centered on the observer, the recession velocity will be c.

Ned Wright has a very useful calculator ... you can plug in your favourite parameters, and get some interesting numbers, from which you can calculate an answer to your question. (Caution: be sure to understand what you are actually asking/trying to find out!)

Hehe, why ruin the experience? There is something to be said for enjoying a moment of slack jawed amazement before attempting to understand it.

The universe is 13.7 billion years old. It's radius is currently estimated at 78 billion lights years, i.e. a diameter of 156 billion LY. So yes, it is expanding much faster than the speed of light. Try this reference:

http://www.astro.columbia.edu/~dave/papers/lineweaver.pdf

Last edited by a moderator:
DrChinese said:
The universe is 13.7 billion years old. It's radius is currently estimated at 78 billion lights years, i.e. a diameter of 156 billion LY. So yes, it is expanding much faster than the speed of light. Try this reference:

http://www.astro.columbia.edu/~dave/papers/lineweaver.pdf

After the results obtained by Cornish, in this paper,

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

he concluded that the observations excluded the possibility of a Universe with a diameter smaller than 78 Gly, but then the popular article that appeared in various webs, for example this, http://www.space.com/scienceastronomy/mystery_monday_040524.html
got the information messed up and proclamed that we live in an Universe that is at least 156 Gly wide. This is wrong, and the radius of the observable Universe is approximately 46 Gly. Read this thread where hellfire clear matters
https://www.physicsforums.com/archive/topic/t-63386_Boarder_of_universe.html

Last edited by a moderator:
meteor said:
After the results obtained by Cornish, in this paper,

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

he concluded that the observations excluded the possibility of a Universe with a diameter smaller than 78 Gly, but then the popular article that appeared in various webs, for example this, http://www.space.com/scienceastronomy/mystery_monday_040524.html
got the information messed up and proclamed that we live in an Universe that is at least 156 Gly wide. This is wrong, and the radius of the observable Universe is approximately 46 Gly. Read this thread where hellfire clear matters
https://www.physicsforums.com/archive/topic/t-63386_Boarder_of_universe.html
we can't observe the whole universe though right? or else we'd be able to see our own galaxy. So how do we know the radius of the actual universe?

michael879 said:
we can't observe the whole universe though right? or else we'd be able to see our own galaxy. So how do we know the radius of the actual universe?
I believe that the assumptions of homogeneity and isotropy allow us to extropolate on what we do observe.

michael879 said:
does anyone know an approximate speed for the expansion of the universe?

The Hubble Constant is about 71 kps/Mpc (kilometers per second per megaparsec) plus or minus 5%. So something 1 megaparsec away from us appears to be expanding away from us at 71 kps and something 2 Mpc away appears to be expanding away at 142 kps, etc...

Right now, the universe is not expanding faster than the speed of light (71 kps is much less than the speed of light which is 300,000 kps), but very distant objects can seem like they are from our perspective (actually, we cannot see them because the light can't reach us due to all the expanding space in between).

or if not, how red-shifted planets are? (with the age of universe would give speed).

I assume you mean 'galaxies', not 'planets'. The planets we can see are not expanding away from us. The expansion of the universe occurs on intergalactic scales, not interplanetary.

And someone told me its accelerating too. did he mean - acceleration? or is it actually speeding up?

Yep, it's a recent discovery that the the expansion rate of the universe is increasing. It's a mystery why it's happening (caused by an unidentified 'dark energy'...where 'dark' means something we don't see, not something evil of course).

michael879 said:
we can't observe the whole universe though right? or else we'd be able to see our own galaxy. So how do we know the radius of the actual universe?

Some would say the universe is infinite (which therefore limits our discussion of a 'radius' on the 'visible universe', i.e., the part we can see based on the speed of light, the age of the universe, and the expansion rate of the universe.)

Phobos in your post 11, I think your statement regarding the 71kps must be multiplied by the size R (approx 1.5 x 10^26 meters) after you convert from parsec. For H equal to approximately 2 x 10^-18/sec and R approximately 10^26 then v = 3 x 10^8 meteres

Last edited:
most distant galaxies are moving away with about 19 times faster than speed of light as a part of expansion of universe. in my calculation, Hubble constant is 74kps per megaparsec, and we have 78 000 of these megaparsecs, since we agree the radius is 78Gly, am i right or wrong?

eha said:
most distant galaxies are moving away with about 19 times faster than speed of light as a part of expansion of universe. in my calculation, Hubble constant is 74kps per megaparsec, and we have 78 000 of these megaparsecs, since we agree the radius is 78Gly, am i right or wrong?

It looks like you forgot to convert light-years (cy) to parsecs, 3.26cy = 1pc, as I recall. So the distance to the particle horizon is ~24,000 Mpc and the speed of objects on the particle horizon would be ~6c ... with these numbers anyway.

As Ruta indicates, it depends on what numbers you use. There is no one fixed speed of expansion, because the rate depends on what horizon distance you choose.

I guess one could argue that the natural choice is the particle horizon---at an estimated 46 billion lightyears.

But another good distance mark would be the surface of last scattering, where the matter is that emitted the Background radiation that we are now receiving. The usual estimate is that the Background has redshift z = 1090.

That is not quite to the particle horizon, but it is not too far from it, since the CMB light was emitted (380,000 years) less than a million years into expansion. The last scatter surface is estimated at roughly 45.5 billion lightyears. 45 point something, near 46.

You can just plug z = 1090 into Morgan's Cosmos Calculator and get the recession rate.
Google "cosmos calculator", put in .27, .73, 71 or what ever the more up-to-date parameters are.
When I use .27, .73, 71 I get that the current distance is 45.5 and the current recession rate is 3.3 c.
Also that the recession speed of that matter when the light was emitted was 56.7 c.

That all makes sense, and is what we're used to, but now with a new Hubble rate estimate of 74, instead of 71, there will be some changes.
By a few percent, but it will take a little while to settle down. The particle horizon distance will be recalculated, likewise the distance to last scatter.
Probably the CMB redshift will stay the same.

Last edited:
The cosmic calculator is still viable, just plug in the new parameters.

no one knows we change our minds every 10 years or so!

meteor said:
After the results obtained by Cornish, in this paper,

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

he concluded that the observations excluded the possibility of a Universe with a diameter smaller than 78 Gly, but then the popular article that appeared in various webs, for example this, http://www.space.com/scienceastronomy/mystery_monday_040524.html
got the information messed up and proclamed that we live in an Universe that is at least 156 Gly wide. This is wrong, and the radius of the observable Universe is approximately 46 Gly. Read this thread where hellfire clear matters
https://www.physicsforums.com/archive/topic/t-63386_Boarder_of_universe.html[The universe is 13.7 billion years old. It's radius is currently estimated at 78 billion lights years, i.e. a diameter of 156 billion LY. So yes, it is expanding much faster than the speed of light. Try this reference:]

Hubbles constant, 74.2 km/sec/mpc does in dicate the recessional velocity relative to our galaxy is a function of distance. However, the discovery of dark energy may mean a much greater expansion near the outer reaches of the universe.

Harrison at page 299 gives a derived velocity of the Hubble sphere as c(1+q). Estimates for the rate of expansion of the observable universe follow from the on-line calculators - but the numbers produced by these calculators would seem to be sharply sensitive to the model used to estimate the size of the cosmic surface at last scattering -

I don't know what you people are talking about. Nothing can move faster than the speed of light. According to the theory of relativity it takes unlimited power to accelerate something even close to the speed of light. Specially something of the size of Galaxies. So either you people are talking nonsense with a strait face or Einstein was an idiot and his theories of relativity are garbage.

Mike

I don't know what you people are talking about. Nothing can move faster than the speed of light. According to the theory of relativity it takes unlimited power to accelerate something even close to the speed of light. Specially something of the size of Galaxies. So either you people are talking nonsense with a strait face or Einstein was an idiot and his theories of relativity are garbage.

Mike

migood said:
I don't know what you people are talking about. Nothing can move faster than the speed of light. According to the theory of relativity it takes unlimited power to accelerate something even close to the speed of light. Specially something of the size of Galaxies. So either you people are talking nonsense with a strait face or Einstein was an idiot and his theories of relativity are garbage.

Mike

What you're talking about follows from special relativity (SR), i.e., it takes infinite energy/force to change v < c to v = c. The result people are talking about here follows from general relativity (GR) where SR holds locally. It's analogous to an ant walking on a rubber band that is being stretched. The ant has a speed relative to the rubber band which has a speed relative to the person's fingers stretching it. Galaxies are considered to be at rest with respect to the rubber band and the photons they emit move at c relative to the rubber band (which means the photons are moving at speeds other than c relative to the emitter and receiver except at the instants of emission and reception, respectively). See "Big Bang Bungled: 6 Common Errors about the Expanding Universe" by Charles Lineweaver and Tamara Davis in Sci. Am. v292, pp 36-45.

migood said:
I don't know what you people are talking about. Nothing can move faster than the speed of light. According to the theory of relativity it takes unlimited power to accelerate something even close to the speed of light. Specially something of the size of Galaxies. So either you people are talking nonsense with a strait face or Einstein was an idiot and his theories of relativity are garbage.

Mike

Yes, it's clear that you do NOT know what we are talking about and it would do you good to read up on the expansion of the universe. What everyone here is talking about is the speed of expansion of the universe, which is a speed that depends on how far away from each other two objects are and has nothing to do with how the objects would be moving relative to each other in a NON-expanding universe, which WOULD be limited to the universal speed limit (what you call the speed of light). The speed of the expansion is in no way limited and is well known to exceed the universal speed limit.

Here is a old "Scientific American" question retooled. If two galaxies are moving apart faster then the speed of light because of the expanding universe and you were in a spaceship traveling the speed of light from one galaxy, traveling to the other, would you ever make it to the other galaxy?

rjohnson103 said:
Here is a old "Scientific American" question retooled. If two galaxies are moving apart faster then the speed of light because of the expanding universe and you were in a spaceship traveling the speed of light from one galaxy, traveling to the other, would you ever make it to the other galaxy?

Some friendly advice. Try not to say "moving" apart. Say that the distance between them is increasing faster than the speed of light.

Are you assuming that the distance between them continues to increase at exactly the same rate indefinitely? (In nature that would not happen so it is an unrealistic assumption.)

On the basis of that unrealistic assumption, the answer is NO, you would never make it from one to the other.

==================================

However because the Hubble rate of distance expansion changes over time according to the Friedman equation there are a lot of galaxies whose light is reaching us today even though it was emitted by the galaxy at a time when the distance to it was increasing faster than c.

"Receding" is a word often used for this distance increasing thing---as distinguished from actual moving. Whether the light can get to us depends. Even if the recession rate is >c it can, under the right conditions, get here.

Try reading the "charley" link in my signature. It is a SciAm article by Charles Lineweaver that discusses this.

Last edited:
rjohnson103 said:
Here is a old "Scientific American" question retooled. If two galaxies are moving apart faster then the speed of light because of the expanding universe and you were in a spaceship traveling the speed of light from one galaxy, traveling to the other, would you ever make it to the other galaxy?

At the risk of repeating Marcus, let me provide an example from GR cosmology (flat, dust-filled model, see Am. J. Phys. 60, Feb 1992, pp 142-146 for details). An object (quasar) with redshift of z = 4.73 has a current Hubble recession velocity of v = 1.2c. At time of emission, the object's recession velocity was v = 2.8c. Thus, you have an example of an entity moving at c (photon) from object 1 (quasar) to object 2 (us) where objects 1 and 2 have always had recession velocities greater than c.

The Lineweaver article was very enlightening, thank you. I was trying to use the caterpillar on the expanding rubber band analogy. If you placed the caterpillar 1/3 of the way across a rubber band it would not matter how fast the rubber band expanded, the caterpillar would always be 1/3 of the way across the rubber band. So any movement toward the end of the rubber band would bring it closer to the that end. I was trying to use this prove the spaceship would make it to the other galaxy even with the space between the two galaxies expanding at >c. I can see now it is not a good analogy.

Last edited:

1. What is the speed of universe expansion?

The speed of universe expansion is measured by the Hubble constant, which is currently estimated to be 73.3 kilometers per second per megaparsec. This means that for every megaparsec (3.26 million light years) of distance, the universe is expanding by 73.3 kilometers per second.

2. Is the speed of universe expansion constant?

No, the speed of universe expansion is not constant. It has been observed to be increasing over time, meaning that the universe is expanding at a faster rate now than it was in the past. This is known as cosmic acceleration, and the exact cause of this phenomenon is still being studied by scientists.

3. How is the speed of universe expansion measured?

The speed of universe expansion is measured using a variety of techniques, including observations of the cosmic microwave background radiation, the redshift of distant galaxies, and the brightness of Type Ia supernovae. These different methods provide complementary data that can be used to calculate the Hubble constant.

4. What impact does the speed of universe expansion have on the evolution of the universe?

The speed of universe expansion has a significant impact on the evolution of the universe. It determines the rate at which galaxies move away from each other, the age of the universe, and the ultimate fate of the universe. A faster expansion rate can also affect the formation of structures, such as galaxy clusters.

5. Is the speed of universe expansion influenced by dark matter and dark energy?

Yes, dark matter and dark energy are believed to play a role in the speed of universe expansion. Dark matter is thought to contribute to the overall mass of the universe, which affects the expansion rate. Dark energy, on the other hand, is believed to be the driving force behind cosmic acceleration and the increasing speed of universe expansion.

• Cosmology
Replies
6
Views
465
• Cosmology
Replies
14
Views
375
• Cosmology
Replies
13
Views
1K
• Cosmology
Replies
20
Views
2K
• Cosmology
Replies
0
Views
158
• Cosmology
Replies
23
Views
3K
• Cosmology
Replies
10
Views
2K
• Cosmology
Replies
15
Views
2K
• Cosmology
Replies
11
Views
1K
• Cosmology
Replies
49
Views
3K