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Suppose you are standing 5 feet (1.5 m) away from me. Then I move 10 feet (3.0 m) further away. Now you are at 15 feet (4.5 m) distance from me.

You say I moved. I say no; the space between us has expanded. You can point me to other things around you that are still at same distance from you. You say if the space has expanded then those things must have also moved away from you.

You are right of course.

If somebody say that there isn't any expansion of space. It's just objects, heavenly bodies whatever that move away from each other. Like when a bomb explode fragments of its shell move away from the center. That’s what’s going on since the Big Bang. What’s wrong in that explanation?

It does go against the well-established claim that nothing that has mass can reach the speed of light. Heavenly bodies—stars, planets, etc.—have mass, so they cannot reach the speed of light given that they start from a lower than light speed.

How can we distinguish the eexpansion of space from objects just moving away from each other? If we plot the expansion backward in time, we see all the objects getting closer to each other and also to a center point so much so that at the very start, the Big Bang, they all converge to a point.

What’s wrong in saying that the Big Bang is an explosion that’s still happening, the heavenly bodies are moving away from the center and therefore also moving away from each other, and that since we see them moving away faster and faster till they reach speed of light and thus we stop seeing them, we can safely deduce that they have reached speed of light and may have gone even beyond?

Why would we say, opposite of what we deduce on basis of observation, and nothing that we observe go against the deduction, that it’s the space that expands, not the object reaching lightspeed and then become even faster?

Is there something in observation that can be pointed out to distinguish between space expansion and heavenly bodies moving away from each other and a center point? If there is nothing in observation to distinguish that then is there something in deduction about it? Like in the thought experiment at the top of this question where you pointed other objects to proof your point that the space didn't expand between us, its I who moved?

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    $\begingroup$ Expansion of space is not distinguishable from objects moving away. It is only a common misconception that there is a distinction. See physics.stackexchange.com/a/505727/180843 $\endgroup$
    – Sten
    Jul 6, 2023 at 8:09
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    $\begingroup$ @Sten You are saying that no galaxy is receding away from us at speed of light or exceed that speed? $\endgroup$
    – Atif
    Jul 6, 2023 at 8:17
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    $\begingroup$ @Sten You said "Cosmological recession rates are not velocities in any relativistically meaningful sense" then theory of relativity do not explain the data and therefore is incomplete / wrong. Our observation tells us that certain galaxies keep accelerating away from us till they reach speed of light and we infer from the available data - no deceleration - that they continue with superluminal speed. This data should be explained. Instead, to justify a theory that don't explain data definition of movement and velocity is changed. You know you can always "win" by redefining what win means. $\endgroup$
    – Atif
    Jul 7, 2023 at 20:59
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    $\begingroup$ @Sten We can choose any definition ofcourse, we just have to be consistent AND explain the observation. There are these two unmovable mountains on which we build theories, or we are supposed to build on that. Ofcourse we can change definitions as we know more (by observation) but then we have to go back and change all theories that use old definitions. You will agree upto this point. Its all about observations. The redshift deduction about its link with acceleration away from us is built on observation. $\endgroup$
    – Atif
    Jul 7, 2023 at 21:32
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    $\begingroup$ @Sten You have to get away from your mathematical mindset of defining your way out of problems. It works only in maths, not in physics. In physics you have to reconcile with observation. Telling fairytale equations don't work. Neither do being inconsistent work. We know by observation that mass accelerating away from us radiate photons that when reach us are redshifted. We observe that even on nearer objects accelerating away from us that nobody claim is moving away from us because of claimed expansion of space. The concept of expansion of space is built upon observing redshifted photons. $\endgroup$
    – Atif
    Jul 7, 2023 at 21:44

1 Answer 1

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In general relativity, expansion of space is not distinguishable from objects moving away.

It is only a common misconception that there is a distinction. This point has been addressed in a number of other answers on this site, by myself and others. Rather than argue it again, I will link a few of my favorites:

However, most readers won't have the general relativity background to assess these arguments on their merits. If you are not sure, then, whether to believe random people on Stack Exchange, take it from eminent cosmologists instead:

The idea of an expanding universe can easily lead to confusion, and this note tries to counter some of the more tenacious misconceptions. The worst of these is the ‘expanding space’ fallacy.

One of the key ideas of general relativity is the importance of distinguishing between coordinate-independent and coordinate-dependent statements. Another is the idea that spacetime is always locally indistinguishable from Minkowski spacetime. Cosmology instructors, books (especially at the introductory level), and students often fall into the fallacy of reifying the rubber sheet; that is, treating the expanding-rubber-sheet model of space as if it were a real substance. This error leads people away from both of these key ideas and causes mistaken intuitions such as that the Milky Way Galaxy must constantly “resist the temptation” to expand with the expanding universe or that the “tethered galaxy” described in Sec. I moves away after the tether is cut.

The answer is: space does not expand. Cosmologists sometimes talk about expanding space – but they should know better.

Expanding space is a very unhelpful concept. Think of the Universe in a Newtonian way – that is simply, in terms of galaxies exploding away from each other.

are not arguments about the theory — everyone agrees on what GR predicts for observables in cosmology. These are only arguments about an analogy, i.e. the translation into English words.

By the way, the last point is why you won't find much discussion of whether space is expanding in academic circles. It's just not a scientific question. That's why the only academic article on this list is an article about pedagogy.

What about faster-than-light recession rates?

There is no need to invoke expansion of space to explain galaxies receding faster than light. Nor do such recession rates violate relativity. This is because the cosmological recession rate is not a relative velocity.

Conceptually, imagine a chain of galaxies that leads to your target galaxy. Each galaxy along the chain has some small velocity relative to the galaxy before it. If you add all of those relative velocities together, that should give you the velocity of the target galaxy, right? However, velocities in relativity add in a special way; see the relativistic velocity addition formula. The cosmological recession rate is computed by instead just adding the relative velocities naively, without properly using relativistic velocity addition. That's why we should not be concerned that it can exceed the speed of light.

Other good explanations as to why faster-than-light cosmological recession is not concerning include:

Also, note that there is not a unique way to define the relative velocity between cosmologically distant objects; see this question for more detail. This is part of why we are fine with talking about recession rates instead of actual relative velocities. There is no unique "actual" relative velocity to even talk about.

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    $\begingroup$ Theory of Relativity has assumption that nothing that has mass can travel faster than speed of light. The theory is built upon that. If anything massive can be shown to move faster than speed of light then the theory just become a fairytale, may be symmetric and beautiful, even logical but dont explain anything about this universe. We have data that show galaxies - that are obviously massive - moving away from each other at speed of light and since we observe them still accelerating - photons from them continue to come redshifted - AND we stop seeing them after that we can deduce they go ... $\endgroup$
    – Atif
    Jul 7, 2023 at 22:02
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    $\begingroup$ ... superluminal. This is observation. It goes against the speed of light assumption in theory of relativity. It makes the theory a fairytale. The theory do not explain this universe even in approximation because the base of theory is gone. Theories are based on assumptions. Its a matter of fundamentals. The theory now explain a fictional universe. So please don't keep bringing it up. Try to think outside that theory and give an explanation. I don't downvote your answer because its clear and logical. It just don't explain anything outside an anti-observation theory. $\endgroup$
    – Atif
    Jul 7, 2023 at 22:11
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    $\begingroup$ Agreed that galaxies moving at superluminal speeds away from us is a deduction, not an observation, but its a deduction based on data and there is no data that go against this deduction. "Again, its not a relative velocity" If you mean relative as a term in theory of relativity then don't keep bringing theory of relativity up because it dont agree with data. Its like being a witness in your own case. Theory of relativity cannot prove itself, duh. If you mean relative in normal sense then how? If a thing move away from me then it moved away relative to me, had a velocity relative to me. $\endgroup$
    – Atif
    Jul 7, 2023 at 22:29
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    $\begingroup$ Please don't mix things up. Ofcourse photons emitted millions of years ago will continue to reach us for millions of years because then the galaxy wasn't moving away from us with speed of light or above. The discussion is about photons generated after the galaxy went superluminal. Come to think of this, if space is not expanding then even those photons will reach us and normally so. The photons always travel at speed of light ofcourse so whether or not the galaxy went superluminal do not matter. We will continue to see the galaxy. There cannot be a Hubble Sphere then. What do you think? $\endgroup$
    – Atif
    Jul 7, 2023 at 22:37
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    $\begingroup$ "Does the fact that two particles ..." How it don't? Aren't all frames-of-reference equally valid? Is there any data that show that one particle do not see the other moving away from it at superluminal speed? Even if there is it dont prove the theory of relativity. One counter example is enough to disprove a theory. It don't matter how many pro example you being then. $\endgroup$
    – Atif
    Jul 7, 2023 at 22:48

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