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After reading about the latest super-massive black hole in Nature 518, 512–515 (26 February 2015), I couldn't help but wonder if the accelerating expansion is a result of mass being lost.

My reasoning is as follows:

  1. If the early universe had a particular (greater) mass than at present,
  2. Then "space-time" could have had a "momentum" (determined by that mass) [1],
  3. And that post-big-bang expansion was being retarded by existing mass [2],
  4. But since then mass has been lost to the universe [3]
  5. Which reduces the (retarding) due to gravity (less mass) [2]
  6. Resulting in the "space-time" of the universe expanding faster [4]

I was never a cosmologist, so please point out which of my assumptions are provably invalid!

[1] Does spacetime have a "mass" value? or What is "Spacetime" made out of?

[2] I'm struggling to remember my undergrad physics - would two particles each with an initial velocity moving away from each other in a gravitational field (relatively) speed up if the gravitational field is reduced?

[3] Major assumption on my part!

[4] Maybe!

So I guess there are two questions here:

A. How confident are we that mass is not being lost in the universe?

and

B. Would such a mass-loss be able to explain the observed accelerating expansion?

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KevinM
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  • Hi first off I would say that the amount of mass-energy in the universe is believed to be conserved, constant through time. So although you can convert mass into energy, you still wind up with the amount of mass-energy in the universe always staying the same. What spacetime is made out off I have no idea, it depends on what hypothesis you believe in. It is just viewed as a place where things happen, like a stage before the actors appear. –  Mar 18 '15 at 19:30
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    Also it is generally accepted that we can only see a small piece of the universe, as the light from further parts has not yet reached us, so estimating how much mass there actually is in the universe is a difficult problem to put a value to. –  Mar 18 '15 at 19:37
  • You say And that post-big-bang expansion was being retarded by existing mass. The big bang is thought as containing all the mass and energy of the universe. All as energy at the start then some energy was converted to mass, as the temperature dropped. that would mean there was no other existing mass. It was all contained in the big bang. I know, it's impossible to get a mental picture of that, that why math is used so much, you spend more time working out equations than trying to visualise it in a commonsense way –  Mar 18 '15 at 19:53
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    These would have been good questions separately... While B is straight-forward to explain, A is certainly going to have an interesting answer. For one we can only describe what's in our light cone from the time of the big bang (the observable universe), whether there's an argument that mass has to be conserved in the observable universe rather than as a whole, or not, is intriguing – Xeren Narcy Mar 18 '15 at 21:42
  • I made a fairly substantial edit. I thought the original question was quite discursive, and contained many sub-questions. If it's not an improvement, please feel free to revert it. – innisfree Mar 18 '15 at 22:52
  • @innisfree hi imo the revision went a bit too far, as a lot of the comments are now harder to relate to the points in the op, but it did need a cleanup, as it was pretty obscure to me (at least!) in places regards. –  Mar 18 '15 at 23:01
  • @XerenNarcy hi good point, if we don't, or won't ever will if expansion acceleration continues) know, we are only left with assuming it's the same everywhere. Totally untestable and no way getting accurate mass value regards –  Mar 18 '15 at 23:12
  • When I said being retarded by existing mass I was referring to the idea of long range gravitational attraction contributing to a "counter expansion" force component. If contained mass were being reduced (lost), this component would be reduced, so whatever force was left, could cause an apparent increased expansion. This only works if expansion is driven, and not static (if there is a non-zero force still driving the expansion), right? If expansion were simply "constant velocity", then such mass loss could not cause an apparent velocity increase. Maybe I'll simulate this in a bounded volume.. – KevinM Mar 19 '15 at 18:40
  • @innisfree I rolled the revision back. It was too radical. It invalidated an answer and changed the meaning of part of the question – Jim Jul 20 '15 at 17:39

2 Answers2

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You ask I'm struggling to remember my undergrad physics - would two particles each with an initial velocity moving away from each other in a gravitational field (relatively) speed up if the gravitational field is reduced?

To answer this part of your question, gravity only works in one way, pulling things together, never allowing them to move away from each other. If the gravity field was reduced, then they would still come together, just more slowly than before

  • True, I misunderstood that. I should have read more carefully /: – Yukterez Mar 18 '15 at 22:39
  • hello, i made a substantial revision to the question that removed that sub-question. it might be reverted... – innisfree Mar 18 '15 at 22:57
  • What I intended was this: If two particles are given a velocity away from each other (in a system with other masses distributed about), yes, their gravitational attraction will cause them to slow down, stop and eventually start moving toward each other (actually, the mass centre). However, in the mean time, if the "other" mass in the system is reduced, the rate of slowing down will decrease. So what will an observer measure? I realise that they will never see an increased velocity, unless there as another driving force that was unaffected by the mass loss. – KevinM Mar 19 '15 at 18:33
  • @KevinM 2 main points that I would make, conservation of total mass-energy of any system is paramount (in the part of the universe know about at least), so the total of the particle's kinetic energy plus their potential energy always stays the same. Secondly, potential energy will always be minimised, given half a chance. e.g. an apple will always fall down if you hold it up and then let it go, so minimising it's potential energy (apologies if this is obvious to you). Best regards –  Mar 19 '15 at 19:17
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  • >> How confident are we that mass is not being lost in the universe? << *

Mass (energy) can be lost in principle: if you convert mass to radiation (which you can, because mass and energy are equivalent), the radiation density dilutes with the growing scale factor to the 4th power because of the redshift, while mass density only dilutes with the scale factor to the third power (because volume is lenght³). So while the total mass provided by matter stays the same even when it thins out while the universe expands, the energy provided by radiation shrinks because the photons do not only get spread out like normal matter but also get their wavelengths increased and therefore their frequency reduced. Because not only mass is equivalent to energy, but also energy to frequency, energy (and therefore mass if you wanted to convert it back later) can be lost.

Yukterez
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  • I don't think you can use an argument of reduced density to say any energy (or mass in turn) is lost, because that would suggest individual photons (as a stand in for radiation) either disappear entirely or lose energy due to redshift (which is problematic). – Xeren Narcy Mar 18 '15 at 22:42
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    but photons do loose energy while the universe expands, the photons which make up the 2.75 K CMB radiation once had a much higher temperature when the universe was smaller. they did not only dilute like normal matter, but also get stretched! Of course they never disappear completely, but the limit of the wavelength goes to infinity as the limit of the scale factor goes to infinity. So in the end you have the same amount of photons, but all redshifted and therefore carrying less energy. – Yukterez Mar 18 '15 at 22:46
  • Ok, I see what you're saying, but it's incorrect to directly compare the frequencies of photons from different reference frames. You can alter the redshift of CMB photons by moving towards or away from their source direction, but that doesn't change the energy for some other observer, so why should any other type of redshift change its energy? – Xeren Narcy Mar 18 '15 at 22:54
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    the photons loose frequency relative to everyone sitting on a comoving coordinate (http://en.wikipedia.org/wiki/Comoving_distance), and from any direction. if you would accelerate to the front to cancel out the redshift in the front direction, the redshift from the back would increase even more than it would anyway. so there's no way around, because you can't accelerate in every direction at once. – Yukterez Mar 18 '15 at 23:01
  • ok, a better example - on the surface of a sufficiently massive object, the gravitational field can be 'tuned' (via the mass) to approximately compensate for the CMB redshift. in such a situation the energy is recovered no? – Xeren Narcy Mar 18 '15 at 23:32
  • to get gravitational blueshift? that might work, but you will need energy to increase the mass (mass and energy equivalence) – Yukterez Mar 19 '15 at 00:39
  • I hadn't considered it that way. Remind me: when mass is converted to energy, the resulting photons don't have any gravitational attraction, right? So all the mass that's converted to energy in every such fission/fusion reaction is causing mass to be "lost"? Makes me wonder if the resulting photons could be accused to driven the expansion of the universe! Pity we can't test this... – KevinM Mar 19 '15 at 18:25
  • Every energy gravitates, also photons do. But photons loose energy while the universe expands, while mass doesn't. That's it. – Yukterez Mar 19 '15 at 21:21