Hubble constant -- early vs recent nonagreement

Factual Questions
1

There’s a problem in cosmology having to do with various measurements of the Hubble constant disagreeing. Basically, those that use measurments based on nearby things get around 74 km/sec/Mpc and those that use measurements from things in the early universe get about 67 km/sec/Mpc. Here’s a blog about it from the Bad Astronomer who explains things better than I can:

It’s currently thought that the universal expansion is accelerating. Cosmologists don’t really understand this, but to give it a name, they call it Dark Energy. Based on this, one would expect that the Hubble Constant in the early universe would be smaller than that of the current universe. Which is what they measure. So why is there a problem? Is the difference too big? If so, what should the difference be?

Or just maybe, all the measurements are correct and the universe is accelerating faster than current theories say.

2

Maybe I’m misunderstanding something, but since the early universe is also further away and therefore since “dark energy” is applied proportionately to how much space is between you and the other object it should be moving faster?

3

That’s embedded into the Hubble Constant. The constant is in the form of X km/sec/Mpc, where the last unit is Megaparsec. What it means is that for every Mpc the object is away, it’s going X km/sec faster. If it’s only 1 Mpc away, it’s moving X km/sec; if 2 Mpc away, 2X km/sec; etc.

My point is if they measure using only data derived from the earliest part of the universe, things weren’t expanding as fast then, so they should get a smaller Hubble Constant. The Constant is not really constant in the accelerating universe. But perhaps I’m not totally understanding it.

4

And in the early Universe, things also hadn’t been expanding for as long. In a universe without the cosmological constant/dark energy/quintessence/whatever the Heaven it is, the Hubble constant would be steadily decreasing with time. In a universe with no gravity, it’d be the inverse of the age, and in a universe with gravity (but none of the whatever-it-is), it’d be decreasing more rapidly than that.

Meanwhile, in a universe composed entirely of dark energy (or whatever), the Hubble Constant truly would be constant. That’s not a terrible approximation for our Universe, which is composed of about 70% dark energy (or whatever), but we do still have 30% of “normal” stuff, and the proportion was even higher in the past.

5

The rate at which the Universe is expanding, known as the Hubble constant, is one of the fundamental parameters for understanding the evolution and ultimate fate of the cosmos. However, a persistent difference, called the Hubble Tension, is seen between the value of the constant measured with a wide range of independent distance indicators and its value predicted from the afterglow of the Big Bang. The NASA/ESA/CSA James Webb Space Telescope has confirmed that the Hubble Space Telescope’s keen eye was right all along, erasing any lingering doubt about Hubble’s measurements.

“With measurement errors negated, what remains is the real and exciting possibility that we have misunderstood the Universe,” said Adam Riess, a physicist at Johns Hopkins University in Baltimore. Adam holds a Nobel Prize for co-discovering the fact that the Universe’s expansion is accelerating, owing to a mysterious phenomenon now called ‘dark energy’.

Open access article at:

https://iopscience.iop.org/article/10.3847/2041-8213/ad1ddd/pdf

6

It’s intriguing to think that there may be something fundamental about the fabric of spacetime that we don’t understand, but the ESA article seems to suggest a possible simple explanation. AIUI, Cepheid variables that are extremely distant may be subject to errors in the determination of their brightness and hence distance because we may not be properly accounting for their light being blended with the light of other nearby stars. Or so the article seems to say.

7

I think you missed something there. That was an potential reason for the Hubble Tension. But these measurements by JWST are to the effect that that wasn’t happening.

OK, can the JWST separate double stars at the distances of these Cepheids? Very unlikely, so there’s still the possibility that that may be throwing the measurements off.

8

Confirmation of the same results by the JWST is indeed significant. But it’s worth noting that its resolving power is not dramatically better than the HST, so it might be subject to the same errors at very great distances.

9

Since this is a topic, I won’t start my own. But can someone confirm to me that galaxies further from us are moving faster than those closer to us?

Is that a correct statement?

And we attribute this to (possibly) dark energy?

10

Yes, that is a correct statement. But not because of “dark energy”. It’s just because space is expanding, and there’s a lot more space to galaxies that are farther away. Dark energy is ostensibly responsible for the hypothesized acceleration of the expansion.

11

It’s a simple matter of geometry. Imagine an ant sitting on a balloon with a diameter of 10 inches. There’s a point A which is 1 inch from him and another point B which is 2 inches from him. Now over the span of 1 minute, the balloon is inflated to a diameter of 30 inches. Everything expands by a factor of 3. So A is now 3 inches away (2 inches farther than it was), and B is now 6 inches away (4 inches farther than it was). So the ant sees that A is moving away from him at 2 inches per minute and B is moving at 4 inches per minute. From the ant’s viewpoint, everything on the balloon moves faster the farther away it is, even though as a whole the balloon is expanding uniformly.

12

How does the idea that things further away are moving faster…because they have always been moving faster, isn’t satisfactory? The expectation is that they should be further away simply because they were initially moving faster.

I know the “horrendous space Kabuey!” is not well modeled by regular explosions that we witness on earth…but in this case it makes sense that things don’t explode outward at the same rate…some are going faster than others at the time of the explosion. Therefore we would expect those things furthest away from us to be moving faster…that’s how they got further away.

Why is this simpler explanation not reasonable?

13

Mainly because the Big Bang wasn’t an “explosion” in any meaningful way that your intuition might picture it. The dynamics of a normal explosion where forces propel the debris outwards at different rates simply do not apply here, where space and time itself are being created.

@markn_1 did a more thorough job than I did in explaining why galaxies further away are receding faster: it’s simply a geometric phenomenon. Think about it again.

The question of why the overall rate of expansion may be accelerating is much more interesting and profound, and that’s where hypothetical dark energy comes in.

14

Because the idea they’re moving faster is wrong.

They aren’t moving faster. The space they’re embedded in is moving faster from our POV. They may not be moving at all compared to the local space where they are.

If indeed the universe had started with a conventional explosion you’d expect various debris chunks to go faster or slower at random. And to be mixed in different directions.

Instead we see direction doesn’t matter at all. Only distance. Which proves via arguments I’m not gonna peck on my phone that it wasn’t a conventional “explosion” of stuff in/through space, but rather an expansion of space itself, carrying largely motionless stuff along with it.

A log floating down a river doesn’t leave a wake. But yet it moves.

15

I think a lot of the confusion for this comes from people picturing all of the matter in the universe scrunched down to one point in space that already exists. Then the big kablooey happens and all of that matter explodes outward into the empty space.

That’s wrong.

The way to look at this is to imagine that you stick a flag at one particular point in space. That flag then never ever moves. It says right where it is, no matter what. Then you go some distance away and place another flag there. This flag also never ever moves and stays exactly where it is.

Then sit back and watch. The two flags slowly move apart from each other. But the flags aren’t moving.

And that is the key to understanding it. The flags themselves don’t move at all. The space in between them stretches.

If you rewind that backwards, then the flags get closer together, because if you reverse time, everything squishes, so much that space itself squishes down to a single point. That point is the “big bang”. It wasn’t a bunch of stuff exploding into space, it was space itself exploding into existence.

Distant galaxies aren’t moving away from us. They are staying pretty much where they are (aside from some local motion). And we are staying pretty much where we are. But the space between us is getting bigger, which makes the galaxies farther and farther away from us even though they aren’t moving.

Take a rubber band and stretch it. At any point on the rubber band, each little part is stretching away from its neighbor by a small amount, but either end of the rubber band is adding up all of those little amounts, so the farther away you are along the rubber band, the farther away you move away from the end of the rubber band.

Distant galaxies aren’t “moving” away faster because they have always moved faster. They are staying where they are. They appear to be moving away faster because there is more space between us and them, which means more stretching. If you have twice as much space, that’s twice as much space to stretch, so the apparent motion is twice as much.

Let’s say you have a balloon. Now you take a magic marker and draw two dots right next to each other. Then you draw a third dot on the other side of the balloon. Blow the balloon up, and the two dots are relatively close to each other, but the third dot is much father away. But any two dots drawn right next to each other anywhere on the balloon won’t move that much.

That is why distant galaxies “move” faster. Space is stretchy.

16

Because if you were in one of those galaxies and looked back at us, it would appear that WE are moving faster away from THEM. And this is true no matter which direction we look. There is no center, there is no “point” from which we (and they) exploded outward. We’re all just embedded in space which is expanding, but in all direction simultaneously.

17

Man…Space sucks…I don’t think I like it.

Thanks everyone who helped explain this…kinda…

18

The explanation that I prefer is maybe a bit dated now, but I made up a pair of overhead transparencies, one with a random pattern of dots, and the other one exactly like it, but blown up by 10%. Put one on top of the other, and you can clearly see the dots on the larger one “moved away” from the center, each one by an amount proportional to the distance… but slide the top transparency over a bit, and you can see the dots instead “moved away” from a different center point.

19

Well, it is a vacuum.
(Mostly)

20

That is excellent. And such a clear explanation of the process and the results. Thank you.

The usual “inflated balloon” metaphor still relies on the person imagining the change happening. With your technique they can see it happening. and change the center point and see it happening differently. From inputs that they know aren’t changing.

I bet that same demo done as a vid animation would be less effective for that reason.