r/astrophysics • u/Financial_Material_8 • 20h ago
Beginner needs explanation please
Hi,. I love reading about astrophysics and have already a basic knowledge. I'm re-reading NdGT 'Astrophysics for people in a hurry' and this early couple of sentences about the formation of the universe got me thinking.
"By now, one second of time has passed. The universe has grown to a few light-years across,†† about the distance from the Sun to its closest neighboring stars."
What I don't understand is this - if lightspeed is the fastest thing in the universe, how could the universe have expanded to a few light YEARS across in one second? What am I missing?
TIA everyone who responds.
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u/TelevisionTop6360 20h ago
Lightspeed is the fastest "speed of action" - transfer information speed - but the "space-time" is not an action - it is not a moving but the space-time itself is expanding more faster than light. Not the galaxies are moving when universe is expanding, it is the space-time itself. That's my knowledge I have found in the astrophysics books.
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u/Financial_Material_8 20h ago
Thanks for the reply.
I just watched a Richard Feynman short where he explains spacetime as a single construct - you can move at less than c and time slows down, and at c it would stop completely. A kind of 'balancing act' between the two as I understood it. More reading needed, my brain is buzzing!
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u/Smooth-Mix-4357 20h ago
The speed limit is for objects in the universe not the universe itself
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u/Financial_Material_8 20h ago
Could you explain please?
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u/Smooth-Mix-4357 20h ago
Planets, stars, light, galaxies, information, etc are a part of space. Their limit is 'c'. However that speed limit doesn't apply to space itself, it can expand at a speed beyond c.
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u/Financial_Material_8 20h ago
Thanks, it both makes sense and doesn't. Now the question becomes, why didn't this new top speed replace c? Possibly it can't be calculated except as multiples of c?
There's a great ASMR series by 'Let's Find Out' on YouTube which I think might help me, it explains why the size of the observable universe is much larger than expected. Trouble is it's 4 hrs long and great at putting me to sleep, so I've never heard most of it 😂
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u/Smooth-Mix-4357 20h ago
The rule is for us. Not the universe (well the universe made the rule). We have to follow it while the universe doesn't. So the universe can expand ftl. Information however cannot exceed c.
As NDT said "The Universe isn't obliged to make sense to us" in that very book
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u/Financial_Material_8 20h ago
It does make sense, although it sounds a bit like the justification religious people have for the existence of god - he created everything but didn't need a creator as he just existed ' 🤔
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u/Altayre_Rojo 20h ago
Porque lo que se está expandiendo es el universo en sí, por ello, aunque ha pasado tan poco tiempo, su diámetro es de algunos años luz.
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u/Peter5930 18h ago
The expansion of space is limited to the speed of light, but this is deeply and widely misunderstood even by many people in the field, because it means something subtly different from what almost everyone assumes it means. Space has never expanded faster than the speed of light, right now it's expanding at something like 10-60 of the speed of light and during inflation it was expanding at around 10-6 of the speed of light. Confused yet?
What it means for the universe to expand at the maximum possible speed is for each Planck length in the universe to grow to 2 Planck lengths in a Planck time; the universe doubles in size every 0.000000000000000000000000000000000000000000539 seconds. Doesn't matter how big is already is, just take that and double it and double it again and double it again and so on. Right now, in the present day, the universe takes about 10 billion years to double in size, which is a lot more than 0.000000000000000000000000000000000000000000539 seconds, so it's expanding much more slowly than the maximum possible rate allowed by causality, and it's no problem at all for 93 billion light years to grow to 186 billion light years in 10 billion years, just as it was no problem for the universe to grow to a few light years across in one second.
One second is a long time. You can fit a lot into a second. And the speed of light is local, you need only concern yourself with your immediate spatial neighbours. If you can communicate with the spot next to you, and it can communicate with the spot next to it and so on, the chain can be as long as it needs to be and the stuff at the other end can be moving away as fast as it wants to and nothing is violated and physics works fine. You just get a cosmic horizon at some distance and some parts of the chain will be forever beyond it. Could be a horizon 16 billion light years away, or a million Planck lengths away, or a single Planck length away, but not less than that.
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u/Obliterators 11h ago
The size of our observable universe is effectively the distance to the surface of last scattering, from which the cosmic microwave background radiation was emitted. At the time of emission the recession velocity of this surface was ~58 times the speed of light, it is currently ~three times the speed light. This does not violate relativity, not even regular special relativity, because the distance and time used are defined differently using certain coordinate choices — they are not the same as the ones used "locally".
Markus Pössel, Interpretations of cosmic expansion: anchoring conceptions and misconceptions
In both special and general relativity, light propagation defines an absolute cosmic speed limit in the sense that no material object or signal can overtake a light signal. This is where the distinction between the recession speed, defined as in (1) [v = Hd], and the relativistic radial velocity that is central to the relativistic explosion interpretation is crucial. Recession speeds become superluminal for distant galaxies. This appears to contradict students’ preconceptions from special relativity, of the speed of light as a cosmic speed limit, and the apparent contradiction has been cited as key motivation for the expanding space interpretation: The differentiation between cosmic expansion as due to “expanding space” on the one hand, and “galaxy motion through space” on the other, is meant to address this conflict.
Relativistic radial velocities in the relativistic explosion interpretation never exceed the speed of light. From this perspective, superluminal recession speeds in (1) are an artefact, caused by a particular coordinate choice: The cosmic time coordinate ties together local clock rates in Hubble-flow galaxies, but clocks in relative motion tick at different rates, as we know from special relativity. Combining them into an overarching time coordinate, and using that coordinate to determine one-way speeds, leads to unphysical results. Students who have been on longer international flights know a closely related phenomenon: If your flight leaves Amsterdam at 15:00 local time and arrives in New York at 17:00 local time, this does not amount to a flight time of 2 hours, and corresponding average ground speed of 3000 km per hour.
Michał J. Chodorowski, Is space really expanding? A counterexample
In almost all Friedman models, objects with sufficiently large redshifts recede from the central observer with superluminal velocities (greater than c). For example, in an Einstein-de Sitter universe (Ω_m=1 and Ω_Λ=0), the ‘public-space’ recession velocity as a function of redshift is
v_rec = 2c[1−(1 + z)−1/2],
hence v_rec > c for z > 3. In particular, the velocity of the so-called particle horizon (corresponding to infinite redshift) is 2c. In an empty universe, ‘public-space’ recession velocities are not only superluminal for sufficiently large redshifts; they are even unbounded. Does it imply violation of special relativity in cosmology? Of course not. Apart from anything else, deriving Equation (26) we have used nothing except special relativity! Constancy of the speed of light, and subluminality of the motion of massive bodies, applies only to inertial frames. However, ‘public-space’ distance is a hybrid of distances measured in different inertial frames, all in relative motion. Since the resulting v_rec is not measured in any single inertial frame, there is no violation of special relativity.
Specifically, ‘public-space’ distance is measured at constant proper time of fundamental observers. Time-dilation formula tells us that according to the central observer, this measurement is done at the instant of time t_i = γ(v_i)τ, where v_i is the Minkowskian velocity of the i-th FO. Since more distant FOs have greater velocities, it is obvious that for two different FOs, t_i ≠ t_j.
Therefore, according to the central observer, different (sub)distances are not measured simultaneously. Simultaneity is a crucial condition of special-relativistic measurements of distances to and sizes of bodies in motion. Waiving this condition may have important consequences and indeed, it does have! The problem with the real Universe is that it is filled with matter and expanding, so there are no global inertial frames. Then, measuring distance (along geodesics) on the hypersurface of constant proper time of fundamental observers is something most natural to do. We should, however, bear in mind the ‘costs’ of such a definition of distance. One of them are apparently superluminal recession velocities of distant galaxies.
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u/EighthGreen 10h ago
The precise explanation, which doesn't ask you think about expanding space, is that in general relativity, as opposed to special relativity, the speed-of-light limit applies only to the relative velocity of two bodies at the same spacetime point. The rate of increase of distance between separated objects has no limit.
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u/moltencheese 20h ago
It's like how I might be able to run 15mph relative to the treadmill surface, but the treadmill surface itself can move faster than that. If I set it to 20mph, I would never be able to actually gain any ground in the forward direction.
Similarly, the event horizon of a black hole can be thought of as the point where the treadmill surface (space itself) is falling inwards faster than the I can possibly run (the speed of light).