What if the Universe expands faster than the speed of light?

[peterraymond]

I may be way off, but at some point in the past I understand there was super expansion where the universe expanded faster than the speed of light. If matter expanded with it it's an example of matter in separate areas moving apart faster than the speed of light. If it didn't the universe has an outer empty envelope surrounding a sub light speed expanding sphere of matter.

If matter did expand with the universe, wouldn't physics appear normal locally everywhere? In that case photons out there should be photons aimed in our direction that are moving away from us. It sounds like we can never observe those photons, but if we could wouldn't they seem to be moving back in time?

And then, maybe the real question. Is there anyway to observe that the universe is expanding, separate from the fact that matter is moving outward? From the matter expansion we infer that the universe has to be expanding and we assume that expansion is uniform in 3D space, but can we show anything like that directly?

OK, to step further into fantasy land, could dark matter and dark energy be somehow tied to the difference between the motion of matter and the motion of space, IE: the expansion of the universe.

 

[jbriggs444]

I may be way off,

Yes. None of what follows is correct.

It is not an expansion in space. It is an expansion of space. It would be more correct to say that things get farther apart without moving. (The English language does not have words to express the reality properly). It is not characterized by a speed but rather by a percentage increase per unit time.

 

[PeroK]

I may be way off, but at some point in the past I understand there was super expansion where the universe expanded faster than the speed of light.

Expansion isn't a speed; expansion is a speed per unit distance. E.g. the current rate of expansion is $70 km/s$ per megaparsec (about 3 million light years).

For any expansion rate, therefore, if you take two points sufficiently far apart, the distance between them will be increasing at a rate faster than the speed of light.

If matter expanded with it it's an example of matter in separate areas moving apart faster than the speed of light.

Yes. There is no limit on how fast space can expand between two sufficiently distant points. Spatial expansion is unrelated to the speed of light.

If matter did expand with the universe, wouldn't physics appear normal locally everywhere? In that case photons out there should be photons aimed in our direction that are moving away from us. It sounds like we can never observe those photons, but if we could wouldn't they seem to be moving back in time?

The current cosmological model has galaxies receding from us faster than light and - a related phenomenon - that light from some galaxies will never reach us.

OK, to step further into fantasy land, could dark matter and dark energy be somehow tied to the difference between the motion of matter and the motion of space, IE: the expansion of the universe.

This is a physics forum, so your fantasy is not a subject that we would discuss.

 

[phinds]

So, what if the universe expands faster than the speed of light?​

The universe would be exactly they way it is now, since that is already happening, although as jbriggs pointed out, it's not "moving" in the sense that you think it is. It's a recession rate, not a proper velocity.

Things at the outer area of our observable universe are currently receding from us at about 3c.

You should read some basic cosmology and a lot of your questions will be answered.

 

[PeterDonis]

Moderator's note: Thread moved to the Cosmology forum.

 

[peterraymond]

I'm not surprised that my questions were as clueless as I suspected they might be. I'd guess a typical result from trying to extrapolate from what one reads in pop-science. It is nice that the answers aren't clueless. In some forums the people answering frequently know no more than the people asking. But then, I went to another thread and did see explanations that didn't seem to agree.

I think that after starting this I need to learn a little before asking any more about the motion of photons inside an expanding space. Should I go to Wikipedia, or is there a better cosmology link that can be recommended?

 

[Orodruin]

typical result from trying to extrapolate from what one reads in pop-science

This is very common yes. It is also the reason it is not recommended to try to extrapolate from popularised sources. Popular science is not intended to teach you science, it is designed to give the public a glimpse of what is going on and inspire a bit of awe while at it. Analogies are never perfect and extemding them to far unfortunately will always result in faulty conclusions.

If you want to do science, then you need to learn what the current theories actually say.

 

[Ibix]

Should I go to Wikipedia, or is there a better cosmology link that can be recommended?

I'm afraid it's quite a long journey to reach the point where you are able to reason reliably about physics in curved spacetime. Davis and Lineweaver is a good start, though.

 

[Orodruin]

Let me just second @Ibix statement and emphasize that Wikipedia is generally not to be considered a reliable or pedagogical source of information. What you need is a textbook and a significant amount of time. You are of course welcome to ask questions here and learn from the answers. We generally do try to adapt the level of the answer to the person asking when possible. However, be careful about extrapolating stuff from popular science.

 

[Bandersnatch]

I think that after starting this I need to learn a little before asking any more about the motion of photons inside an expanding space. Should I go to Wikipedia, or is there a better cosmology link that can be recommended?

Myself, I think Wikipedia is a-ok. Incorrect statements are generally absent in articles on cosmology. And if there are any debatable nuances, they will be lost on a new reader anyway. Its downside is mainly that there's too much information, scattered among many pages, with little thought given to pedagogy. Without a more structured approach it's easy to get overwhelmed, focus on bits of minor relevance, or overlook the important ones. But spending a few evenings wiki-diving can at least provide a general view of the landscape, to help one orient themselves by.

As for further reading, there's Ned Wright's tutorial: https://www.astro.ucla.edu/~wright/cosmolog.htm
Wayne Hu's website is an excellent resource on everything relating to the cosmic microwave background radiation: http://background.uchicago.edu/index.html

Also, I really dislike comments telling people to go and read a textbook. So I'm going to tell you to go and read a textbook. If you can, get your hands on on Andrew Liddle's Introduction to Modern Cosmology (anything past 1st edition, for fuller picture). You'll find it very clearly written, able to be read almost like a popular publication. Understanding the maths included in the text, while recommended, can be treated as an optional exercise that clarifies the picture. The level of complexity is similar to that in Wright's tutorial above.

 

[peterraymond]

Thanks for the responses and references. I haven't gotten very far yet and am not making any statement with confidence. I skimmed Wikipedia on red shift and found a source for the highest observed red shifts that documents relative motion of much more than C.

It seems like a key concept is understanding the consequences of each observer existing in an individual space time. Is any observer, that is not acclerating, stationary in an individual space time?

One thing I think is helping me is picturing two objects separating at a relative velocity greater than C and a series of several intermediate observers: 1 through n, at proportional spacing and speeds. I understand that maybe I shouldn't be using the word velocity, but say we are one end of the chain and the other end is moving away from us at 2C. Ignore for now how that happened, but expansion will work. If you have light shining towards you from the far end it will of course get to the intermediate observer n, since the relative speed of motion is much less than C. The light will go past that observer at C, of course, and so will easily reach observer n-1. This continues until it reaches us at the other end of the chain.

Whats great is that if it's a 1 second pulse it will have a certain number of cycles. When the pulse reaches us the number of cycles will be the same, but the wavelength will have shifted, as will the 1 second length of the pulse. The start of the pulse took less time to travel then the end. The separation was greater when the end of the pulse was transmitted.

Now build a gun of some type and fire masses in opposite directions at more than 0.5C and have one of them send light back. I think that this is in theory possible and again has objects moving apart at more than C. Here too the light will get from one end to the other.

It seems that light can go from anywhere to anywhere without traveling faster than C.

I think I'm going to go back to the red shift article and try to understand if we have any direct way to distinguish between relative physical motion and expansion. I'm leaning towards no. I also think I'm going to try and ignore for now the effects of gravity and spend more time reading the references. My Newtonian brain is also grappling with the conservation of energy implications of expansion. Expansion has consequences, but also has to have causes and limits.

Thanks.

 

[jbriggs444]

Thanks for the responses and references. I haven't gotten very far yet and am not making any statement with confidence. I skimmed Wikipedia on red shift and found a source for the highest observed red shifts that documents relative motion of much more than C.

If red shift could always be ascribed to relative motion, this would be meaningful. But red shift occurs from other causes (expansion of space) as well. So no superluminal proper motion is involved.

It seems like a key concept is understanding the consequences of each observer existing in an individual space time. Is any observer, that is not acclerating, stationary in an individual space time?

It is all one space-time. There is no such thing as motion relative to space time. There is only motion relative to other physical objects or to agreed-upon coordinate systems. Even then, motion relative to physical object is only locally significant.

One thing I think is helping me is picturing two objects separating at a relative velocity greater than C and a series of several intermediate observers: 1 through n, at proportional spacing and speeds. I understand that maybe I shouldn't be using the word velocity, but say we are one end of the chain and the other end is moving away from us at 2C.

Right. You should not. Relative motion is only locally meaningful. The other end is not moving away. The distance in between is increasing.

Ignore for now how that happened, but expansion will work. If you have light shining towards you from the far end it will of course get to the intermediate observer n, since the relative speed of motion is much less than C. The light will go past that observer at C, of course, and so will easily reach observer n-1. This continues until it reaches us at the other end of the chain.

If the distances were not increasing, this could be the case. But the distances are increasing.

Say we are standing 10 distance units apart. You shoot a light beam at me and it crosses one distance unit. In the time that this takes let's say that expansion has doubled the remaining distance.

Now we are standing 20 distance units apart. Your light beam still has 18 distance units to go. You wait for it to cross another one distance unit. But expansion has doubled the remaining distance again.

Now we are standing 40 distance units apart. Your light beam still has 34 distance units to go.

At this rate your light will never reach me. I am standing beyond your cosmological horizon.

This is sometimes considered in the context of ants crawling on the surface of an expanding balloon.

Say it is a 12 inch balloon expanding linearly at 100 miles per second. An ant starts at the "North Pole" and crawls southward slowly. He cannot keep up with the expansion at first. But it turns out that this ant can eventually reach the south pole. It takes an absurdly long time. The result relates to the sum of a harmonic series. You consider the ant's journey step by step. He makes it at least 1/x of the way, then he makes it at least 1/2x of the way, then he makes it at least 1/3x of the way. The sum of this series diverges -- which means that the ant can always make it all the way there.

But if instead of a linear expansion rate it is an exponential expansion rate then if the ant cannot keep up with the expansion at first then he will never be able to keep up and will never reach his destination.

The expansion of the universe is of the exponential sort.

 

[Orodruin]

But red shift occurs from other causes (expansion of space) as well.

I think it is worth pointing out that all ascribing of such causes to different effects are coordinate dependent and that many statements and separations only make sense relative to a predefined set of coordinates. As such they do not really hold physical significance but are rather helpful aids to think about things. However, this is likely beyond the OP’s current level.

 

[peterraymond]

When I said each observer has their own space-time, I see I used the wrong words, but I was just trying to say that everyone appears stationary to themselves.

Davis - Lineweaver, have a Scientific American article that is more approachable and can be found outside the daunting SA paywall at http://www.hep.fsu.edu/~wahl/artic/varia/BigBangMar2005/BigBang0503.htm

If we shoot an observer and an object towards each other at 0.6C each, they appear to us "stationary observers" to have a 1.2C closing speed. However, the observer clearly has to see see the object approaching at less than C. I found exactly this example in my college physics book. And the fairly straightforward equation that yields 0.88C, but is the observer's red-shift still 1.2C?

Also, if we take a snapshot of the observer's equipment as they sail by, we see that their stopwatch has slowed down and their ruler has gotten shorter. There wasn't an equation supplied for either of those effects.

OK, I have to say this: a parsec is strange. The Earth's orbit isn't even round. Sticking to MKS is great since there E = MC^2 has no conversion factors. There H0 = 2.3 E-18 m/s/m, (2.3 m/s/exameter) which also seems to more directly show how diffuse expansion is.

Is there general agreement on an expression for H(t) predicting expansion in the future?

How about a theory that is confirmed by the expansion vs time, as opposed to one that tries to explain what we see?

 

[PeterDonis]

And the fairly straightforward equation that yields 0.88C, but is the observer's red-shift still 1.2C?

Redshifts aren't speeds. The redshift of one object as measured by the other is the redshift corresponding to a relative velocity of 0.88c.

if we take a snapshot of the observer's equipment as they sail by, we see that their stopwatch has slowed down and their ruler has gotten shorter.

What you actually see in "snapshots" (i.e., light images) is more complicated than that. When the observer is moving towards you, their watch appears in the light images you see to be ticking faster, not slower; the "ticking slower" is not a direct observation, it's a calculation after allowing for the effects of changing light travel time because the observer is moving relative to you.

As far as the appearance of the ruler, it won't appear shortened, it will appear rotated. Look up Penrose-Terrell rotation.

There wasn't an equation supplied for either of those effects.

You shouldn't expect one from the Davis & Lineweaver article since it is not a teaching resource for special relativity, which is what the effects you describe are from. It is talking about the expansion of the universe, which requires curved spacetime, i.e., general relativity, not special. You will not have any success trying to interpret our observations of an expanding universe in terms of special relativity, since SR is not the correct thery when spacetime is not flat.

a parsec is strange

It's a perfectly well defined unit, chosen because measuring parallax was one of the first ways we had of getting data on the distances to nearby stars. The definition in no way requires the Earth's orbit to be exactly circular.

Is there general agreement on an expression for H(t) predicting expansion in the future?

How about a theory that is confirmed by the expansion vs time, as opposed to one that tries to explain what we see?

I'm not sure what you are getting at with these questions, but they don't sound like you are trying to understand our best current model of the universe in cosmology, which is what this forum is for.

The short answer to your questions as you ask them is that of course our best current model predicts what will happen to $H(t)$ in the future, and of course it's not possible to have any such prediction confirmed since we have only been making cosmological observations for about a century, i.e., about 1 ten millionth of the age of the universe, and we should not expect to see enough of a change in $H(t)$ on such time scales to be able to compare it with any predictions.

 

[Ibix]

If we shoot an observer and an object towards each other at 0.6C each, they appear to us "stationary observers" to have a 1.2C closing speed. However, the observer clearly has to see see the object approaching at less than C. I found exactly this example in my college physics book. And the fairly straightforward equation that yields 0.88C, but is the observer's red-shift still 1.2C?

As PeterDonis says, the problem with all this is that it's special relativity, not general. The results of special relativity are - wait for it - special cases. They apply in flat spacetime, and all cosmological effects stem from the curvature of spacetime (even the so-called flat universe is only spatially flat - spacetime is still curved).

Also, if we take a snapshot of the observer's equipment as they sail by, we see that their stopwatch has slowed down and their ruler has gotten shorter. There wasn't an equation supplied for either of those effects.

A lot of sources (me included if I'm not being careful) don't make a clear distinction between what you see and what you calculate after the fact. When things are traveling at near lightspeed this can be a significant difference, because where the near end of an object is changes in the time it takes light from the far end to overtake it. Only once you correct for that do you determine that length contraction and time dilation happen - you don't see them directly.

Sticking to MKS is great since there E = MC^2 has no conversion factors.

...um, a conversion factor is exactly what the $c^2$ is. Using units where it's 1 is a good idea in a lot of areas of physics, which is why you'll often see particle masses quoted in MeV, an energy.

There H0 = 2.3 E-18 m/s/m, (2.3 m/s/exameter)

And that's why. Who needs all those $10^{-18}$s cluttering stuff up? Pick a better unit.

Is there general agreement on an expression for H(t) predicting expansion in the future?

How about a theory that is confirmed by the expansion vs time, as opposed to one that tries to explain what we see?

Over the time we've been actively measuring it the Hubble constant hasn't had time to change appreciably, and with current technology it'll be a good few tens or hundreds of millions of years before we can detect changes big enough to test predictions of the future behaviour of $H$. The past is all we've got for now.

 

[peterraymond]

I'll say and/or admit a few things.

To hopefully simplify the answer my special relativity question was intentionally posed so that things like expansion of the universe and gravity were excluded. It's actually great that even special is more complicated than my mental model.

I guess I'm relieved that the red shift in the example corresponds to 0.88C. Also, I was saying that equations for change in length and slowing of time were missing from my physics book, Halliday and Resnick. But that wasn't a complaint. They were not trying to really teach even special relativity, but more just pointing out where equations are low speed simplifications.

My questions about H(t) hoped that there were answers. There have been many different ways that theories, including special and general relativity, have been confirmed in what seem to me some reasonably subtle ways. For expansion the holy grail might be a theory that required a universe that included quantum mechanics. No, I'm not proposing that. I just hoped that there was some way to confirm short of waiting.

We see that expansion is accelerating. A current estimate for the particular current dH/dt would be interesting and also... Well, I don't really know how to correctly ask this, but something like H as a function of space-time.

Warning, save some time, past here is 90% off subject:
I like the fact that all the forms of energy described my Maxwell's equations are in some real sense equal, MKS incorporates this and Maxwell's equations also let us calculate C, yet Maxwell had no idea that special relativity was coming along.

And that's why. Who needs all those s cluttering stuff up? Pick a better unit.

I did, the exameter. It's, for reference, a little less than 106 light years. Using the roughest numbers, H = 2 m/s/exameter and it's 250 exameters to the center of the milky way. Can I say that between here and there space is expanding at around a trivial 500 m/s?

There are scientists using trigonometry to calculate celestial distances and I recall that we are accurate to around 1 significant digit measuring from here to the center of the Milky Way. If I were doing that I would not expect to use minutes of arc, since th = sin(th) = tan(th) for these distances if we use radians. I expect I'd use micro, or nano-radians. And also, many observations over time using the actual position on the Earth's orbit expressed in meters or km.

A Parsec just seems like remnant left over from an earlier age. Like feet and miles. A Light year is also really not that much smaller than a parsec and more generally understood, even if it too is in part earth-centric.

I do apologize for this subject being a hijack though, so thank you for reading and I suggest that this whole units rant should suffer a quiet death.

 

[PeroK]

To hopefully simplify the answer my special relativity question was intentionally posed so that things like expansion of the universe and gravity were excluded.

Those assumptions don't leave much room for cosmology!

 

[peterraymond]

Baby steps!

 

[PeterDonis]

I suggest that this whole units rant should suffer a quiet death.

At this point this thread is being given the same fate. Thread closed. In future threads, please stick to one topic per thread, and please do some basic reading in a valid source (textbook or peer-reviewed paper) before posing questions.