Naked Science Forum
General Science => General Science => Topic started by: chris on 03/11/2017 22:33:40
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Someone asked me if there are galaxies in the Universe retreating from us at faster than light speed and, if so, how could they be breaking the cosmic speed limit.
I argued that if space itself expands between two galaxies at faster than light rates then the two galaxies would appear to move apart but without actually having to "move" at all.
But is this a valid argument? And does it happen?
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Well gravity tends to draw particles together and we say that alters spacetime. It also involves length contraction. So the converse, expansion, could be said to involve a length extension. I have no idea what that would mean for time.
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The apparent superluminal velocity of galaxies is due to miscalculation: failure to include relativistic-effects ... http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/Superluminal/superluminal.html
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I heard one astronomer put it like this: You can't accelerate matter to speeds faster than c. But there is no such limit on the stretching of space.
So in principle, the cosmic expansion of the universe could result in galaxies moving away from us faster than the speed of light. They would be invisible to us because light from them would never reach us.
With an accelerating expansion of the universe, there may be galaxies that were visible in the past, but not visible now (or galaxies that are visible now, which will become invisible in the future).
See short description: https://en.wikipedia.org/wiki/Hubble_volume
longer description: https://en.wikipedia.org/wiki/Metric_expansion_of_space
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I heard one astronomer put it like this: You can't accelerate matter to speeds faster than c. But there is no such limit on the stretching of space.
So in principle, the cosmic expansion of the universe could result in galaxies moving away from us faster than the speed of light. They would be invisible to us because light from them would never reach us.
With an accelerating expansion of the universe, there may be galaxies that were visible in the past, but not visible now (or galaxies that are visible now, which will become invisible in the future).
Thanks Evan, and phew, I have said the right thing.
Next question. Is there evidence that this is happening?
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The best way to think about it is that the Doppler shift itself indicates expansion. When people argue that this could be caused by something else then how do they explain that some galaxies are blue shifted?
http://curious.astro.cornell.edu/about-us/104-the-universe/cosmology-and-the-big-bang/expansion-of-the-universe/626-how-is-it-proved-that-the-universe-is-expanding-intermediate
If we plot constant acceleration then our graph is a straight line with an equation of the form y = mx + c. With m being acceleration, the gradient, x bring time and c being initial velocity. The Hubble plot is not quite a straight line. This in itself is evidence of an accelerated expansion.
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You can calculate in simple terms when a linear acceleration of space will reach the speed of light. If we take vi to be our instantaneous velocity then vi = at + v0. We can then change this by setting initial velocity to zero and restating as c = |at|. For any constant acceleration there is only one value for time that is a solution. This can be thought of in the same terms as the product of frequency and wavelength in determining light speed. If we set time to infinity then the result is undefined.
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The apparent superluminal velocity of galaxies is due to miscalculation: failure to include relativistic-effects ... http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/Superluminal/superluminal.html
Doesn't this apply to galaxies that are moving towards us, and across our line of sight?
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Is there evidence that this (unobservable, >c galaxies) is happening?
Cosmologists have tried various methods, and come up with wildly different answers.
- Some estimates have put the size of the actual universe to be immensely larger than the observable universe, and the vast majority of galaxies are moving away at >c, and are beyond our view.
- On the other hand, some have suggested that the actual universe is smaller than the observable universe, and what we see as really distant galaxies are repeated images of closer galaxies, but seeing them when they were younger. They have looked for hints of this in repeated patterns in the Cosmic Microwave Background Radiation (CMBR), but not really found any.
So at this point, I would have to say that it is largely a matter of opinion.
See: https://en.wikipedia.org/wiki/Observable_universe#The_Universe_versus_the_observable_universe
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We can demonstrate mass impacting space-time; gravity. We can show where lowered mass density will cause space-time to expand and/or increased mass density will cause space-time to contract. GR mathematically describes this and experiments have demonstrated this.
On the other hand, there is no experimental proof that space-time can lead mass to do anything, without space-time first being induced by mass, elsewhere. All conclusive experiments of space-time changing, start with mass first.The main reason is, nobody has even made theorized dark energy in the lab, so they could conclusively run an experiment of space-time leading, without mass coming first. The being said, dark energy in the lab, itself, would require experimental equipment made of matter, and not just massless invisible equipment. It comes back to mass leading; big boom. Changes in space-time is an affect not a cause, based on the preponderance of direct experimental proof.
That being said, let us assume dark energy and then space-time, can lead matter and mass for the sake of argument. Say we see a galaxy 1 billion light years away. What we see is the light from that galaxy that was emitted 1 billion year ago. This light defines the state of the galaxy, 1 billion years ago. This light does not reflect what the galaxy is doing today, since it took the light 1 billion years to reach us. Therefore there is a time delay between the galaxy event and our observation. We see history and not news. If an alien race has sent the signal, and it took a billion years, the engineers who made the tools would be dead for 1 billion years.
If we assume this light took 1 billion years to reach us, and the universe is expanding due to dark energy and space-time leading, then that light will continue to red shift, for 1 billion years, after it is released due to the universe expanding via space-time. Energy is not exempt from change if universal space-time is expanding and leading.
What that means is that secondary massaging of the original output, by space-time, could make the galaxy appear to move faster than C if you assume this all due to the galaxy. The galaxy could be moving at 50% C , 1 billion years ago, when it outputted light. Then the universal space-time expansion takes over and red shifts the light signal to say double, by the time the signal reaches us. Now the galaxy looks faster. The current logic is conceptually flawed.
Many people sense a problem and some have attributed this to gravitational lensing. However, the problem is connected to having space-time lead, since space-time impacts matter and energy in a continuous way, such that the original output will never the same, if there is any time delay.
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Last night I attended Brian Cox's travelling show in Sydney, and he addressed this very question.
His view was: there is evidence that the universe is much larger than the visible universe, and perhaps infinite.
PS: I quite enjoyed the show, with Brian providing most of the science, and Robin Ince providing most of the comedic relief (like the "Infinite Monkey Cage" podcast). There were lots of spectacular astronomical images. There was also a twitter account for raising questions at intermission; Brian addressed some of them in the second half of the show.
However, I do wonder if they were conducting an experiment on the audience, as several times the same statements were repeated, seemingly aimed at provoking the more pedantic members of the audience (such as myself).
I won't mention them here, as I don't want to spoil it for other people booked in to see the show (or to bias any research they may be conducting...).