Right, I’ll try to keep this as brief and as simple as I can in the hope that any correct answers can be delivered in the same fashion.
I’ve watched a few astronomy documentaries recently, and they all they all more or less say the same 3 things, which I consider to be contradictory;
The speed limit of absolutely everything (lets ignore those pesky recent neutrinos for now).
The age of the universe is in the region of 13.7 billion years old.
The size of the observable universe is 46 billion light years.
So what gives? How can the size of the observable universe (let alone the possibility that there’s a great deal more out there we can’t observe) be so much greater than the maximum speed limit the cosmos allows it to be?
There’s no upper limit on how fast the Universe itself can expand. Early in the Big Bang, the “fabric” of the Universe was expanding at many billions of times the speed of light. The speed of light is only a limiting factor for objects traveling in the Universe, not for the Universe itself.
Some discussion here.
What I don’t get, despite following every cosmology thread on this board, is how we came up with an estimate of the size of the universe. We can’t see past the big cliff at the 13.7 billion LY mark, and we didn’t do it my measuring its curvature and using that to figure out where it closes back in on itself. The probes tell us it’s a flat disc. So how did we come up with the various figures I’ve read, which range from about 46 – 152 billion LYs?
When people refer to the size of the Universe, they mean just the observable Universe. The whole Universe might, for all we can tell, be infinite, and if it’s finite, we have no idea whatsoever of what its size is, beyond that it appears to be larger than the visible portion.
If we see something at 13.7 bly away, we know where it was 13.7 billion years ago. That means it’s had 13.7 billion years to continue moving. (Or,it might be more correct to say that the universe has been expanding between us and it for that long.)
There’s also research that suggests we can see the effects of gravity from things beyond the visible universe.
So far as we can tell (and its been studied, researched, tested, and accrued more evidence than almost any other phenomena) 186,200 miles per second is the fastest anything can travel within spacetime itself. It is a universal constant denoted as c, and is colloquially referred to as the “speed of light.”
That’s what the evidence seems to point to, yes, however…
…spacetime itself is expanding. This is different than c. Imagine a lake that formed 13 years ago, and not too long after, hundreds of boats arrived on the lake, all in random positions, cruising about at their own pace; but each boat’s motor is incapable of going any faster that 186 mph.
All the while a deluge of water is continually pouring into the lake. The lake is constantly swelling in size and the boats recede relative to each other, because the area of the lake is increasing.
So there’s what we have. Replace boats with galaxies; the boats’ top speed as c; the lake with space; the deluge of water as the inflation of the universe.
Now we can see that the size of the universe is what it is because space, itself, is expanding, and this is independent of how fast stuff can move within it. And its current size is misleading if you figure the “lake” can only grow at 186 mph in the 13 years its been around.
Where’s this “deluge” of space coming from? I dunno.
To put it yet another way: You have to remember that when they say that the universe is expanding, that galaxies are moving away from each other in all directions, they are not traveling thru intergalactic space the way that the planets are traveling thru inter-solar space as they orbit the sun. Space itself, is expanding, and the galaxies are going along for the ride.
Which is what pretty much everyone expected would turn out to be the case. All of the speculation was just because nobody was 100% sure, and anyways speculation is fun.
Unclear question. Galaxies are a lot bigger… Galaxies are also compact enough not to fly apart as the cosmos expands. The gravitational attraction holding galaxies together overcomes the expansion of space. This applies to small clusters of galaxies; theAndromeda Galaxy, for instance, is moving toward the Milky Way, even though the space between them (us!) is expanding.
But I’m not quite sure what you actually were asking?
The problem I’ve always had with this explanation is that the phrase, “space itself is expanding” is meaningless. I’ve never had it explained properly. Yes there are analogies like the boats on a lake, or raisins in a loaf of expanding bread… but space doesn’t have a medium. Yes, there is the “quantum foam” and whatnot, but by and large, space doesn’t HAVE a fabric. So what is there to expand?
And, I’m pretty sure that if you do the calculations, two galaxies on “opposite sides” of the observable universe are moving faster, much faster, than the speed of light, relative to one another. We explain it by saying that the space between the galaxies is expanding, producing these effects… but again, the measurements/calculations show relative velocities between bodies greater than c, which bothers me.
There’s a bigger paradox out there. According to our best estimates, there are stars in the universe that are older than the universe itself. Obviously, this can’t be true. But it points out that our current methods of measuring the ages of astronomical objects must be flawed. Either some of our assumptions or some of our measurements (or both) are wrong.
Little Nemo, I believe that the paradox you’re talking about has been resolved. The estimates of the age of the universe and the age of those stars have both been revised, and it’s no longer true that the estimated age of those stars is older than the estimated age of the universe. I assume this is what you’re talking about, or are you talking about something else?:
I’m an amateur cosmologist and no mathematician so I don’t know any more than you. I understood the Universe was calculated to be 16.5 - 23 billion light years in size so 46-152LY is news to me - but whadda I know.
I suppose the period of hyperinflation (faster than C) after the Big Bang could have gone on longer thus producing a Universe 46 etc LY across. I simply don’t know.
I don’t know where you got the flat disc from. Einstein pictured the Universe to be saddle shaped which he calculated from the geometry of parallel lines meeting. Or so I vaguely thought - happy to be corrected.
Correct. Science faces challenges when looking deeply into the beginning of the Universe.
There was no light for 300,000 years because the primordial atoms were clumped together from the Big Bang. Then photon decoupling occurred and light escaped. So, there is no ancient light for us to observe earlier than 300,000 years and thus we are blocked from seeing the earliest Universe.
However what we could possibly view are the gravity waves at earlier times generated by random clumps of matter. That would give us a description of the very early stages.
To do this we need to launch a gravity detector and that is not practical just yet. I understand such a device would be made of iron, be L shaped, and measure a kilometre or more on each side. Gravity waves would twist the L and give us information.
Space doesn’t have a physical medium (e.g. is isn’t made out of matter, like the atmosphere, or energy, like radio waves) but it is a medium, sometimes referred to in cosmology and relativity physics as a “plenum”, essentially a manifold in which all normal matter and energy are embedded and which describes the topological relationships between them, e.g. gravitational and magnetic attraction.
Although matter and electromagnetic energy (and presumably gravitational energy) can only move at speeds up to or at c, there is no limit to how fast the intervening space can expand or contract, and thus, how fast matter that is embedded in one local reference frame of the plenum can be displaced relative to another non-local reference frame. This is not just theoretical mumbo-jumbo; we’ve actually witnessed this effect, as predicted in general relativity, in the anomalous precession of the orbit of Mercury, and it is applied routinely in correcting the position of GPS satellites (which have to be measured to a high degree of precision).
As far as we can observe, space expands at a constant rate per unit measure, but the further away things are, the greater the multiplier (rate x distance) so that objects that are very distant are being displaced at great speeds even if their proper motion should be very small, as codified in Hubble’s law. Also, the electromagnetic energy–light–emitted from those objects is being stretched out as the intervening space stretches, resulting in the redshift, eventually disappearing below observable frequencies entirely. The light of objects we see that are currently at the edge of the observable universe–approximately 46 Bly away–was emitted 13.7 By ago but at the time the objects were only ~40 Mly from our current reference frame. That may seem hard to wrap your head around, but that is the consequence of dealing with cosmological scales where the speed of light makes causality not anything like instantaneous.
As for what makes up the “fabric” of space, the answer is that we don’t know and we can’t measure or even directly observe it. We can only observe the emergent properties of real matter and energy that are embedded in it. However, we know that these underlying properties are real because they match our predictions based upon the models of general relativity to a very high degree of precision. If this seems like a case of being able to say how the animal behaves, what it eats, and where it sleeps, without actually being able to identify the animal, well, that’s a pretty apt analogy. But that is the best we have, and while it cold be seriously wrong, any model that replaces it is going to have to provide essentially the same properties that match our current predictions.
Just for clarity - are you saying that local space expands faster than c when we observe the precession of Mercury? I thought it was simply special relativity and gravitational lensing.
