Comment by tdfirth | Hacker News Reader

Comment by tdfirth | Hacker News Reader

This is probably really dumb but I'm interested to hear people's reactions to yet another amateur dark energy theory.

Could the observation that the universe is expanding actually be the result of a sort of gravitational lensing and not the result of space expanding or bodies moving through space?

My rough line of thinking is:

  - We believe that gravity has a warping effect on spacetime.
  - We believe that this warping can distort light.
  - When we observe light from distant galaxies, it must first exit the gravity well of the source, traverse the universe and then make its way down into our gravity well.
  - Therefore, the light must have been subject to some distortion. It traversed two regions of 'stretched' space time and so became 'stretched' itself.
  - This 'stretching' is interpreted as redshift and is indistinguishable from if the light source had just been moving away quickly.

Naively, I would have thought this interpretation explains a couple of things quite well:

  - Light would be redshifted in every direction as every bright and distant source (i.e. galaxies) resides in a large gravity well.
  - It would explain a lack of redshift in nearby objects as we sit in a broadly similar part of the Milky Way's gravity well. As such the relative lensing effect would not be apparent.

Anyway, I obviously have no idea what I'm talking about and someone will have thought of this before - however I'm interested to know why the theory doesn't work.

Most likely I don't really understand gravitational lensing and it wouldn't distort the wavelength of light as I suggested. Alternatively, perhaps someone has done the maths and the impact of any such effect does not align with observations.

edit: formatting

Light is indeed redshifted as it climbs through a gravitational well, but it is also blueshifted again when it travels down the gravitational well of our own galaxy. For distant galaxies, the total effect is negligible compared to the doppler redshift due to their velocity. Also, when you look in any direction in the sky, the redshift increases with distance, irrespective of galaxy or cluster masses. So the further they are away, the faster they are receding from us. This is also exactly what the Hubble constant measures.

Cool, thanks for the reply! I hadn't considered the blueshifting part, that makes a lot of sense though.

I also hadn't heard of Hubble's constant, which I am now learning about :). Very mysterious indeed.

> the further [galaxies] are away, the faster they are receding from us

I've heard this repeated many times as a surprising discovery. Why is this surprising? Let's say you have some number of objects moving at different speeds (relative to Earth). After enough time, the objects that move fastest should be farthest away, by the simple definition of speed!

What am I missing?

Substitute "speed" (really, "velocity") with "acceleration" and your statement is true. So, you're pretty close in your reasoning - if everything is accelerating at the same rate, then things further away from us should be moving away from us faster.

So here's the problem: maintaining a constant velocity requires no outside force, but acceleration does. So, we're seeing everything accelerate away from us.... but why? Where does that force come from?

Ah thank you :).

One more question: How do we know that each galaxy is actually accelerating and not moving at a constant speed?

I'm seeing some circular logic. As I understand it, we think they are accelerating _because_ we see that galaxies that are farther away are moving faster away from us.

But we just agreed that we expect to see this same observation even if each individual galaxy maintains constant velocity. Apologies. It's a bit late. I'm sorry if I'm missing something incredibly obvious.

between 1970 and now, if you look at the same far away galaxies, they are receding faster now than they were in 1970.

^_^ thank you

Wait... Is this correct? Do you have a source? This is the first that I've heard of this being directly measurable.

Dumb question: is it actually accelerating or are we slowing down?

Possibly dumb answer: If everything else was moving at a constant velocity and we were slowing down, wouldn't the things on the other side of us appear to be accelerating towards us?

My understanding is that basically everything is moving away from us (presumably our reputation precedes us).

They are all accelerating away from us, in every direction.

do you mean slowing down in how we experience time?

Amateur so I’m sure this is a stupid idea. What if they’re not accelerating away from us but space time itself is growing (because of the original Big Bang) and taking everything else along for the ride on top of the original exploding outward, resulting in acceleration of acceleration? Probably it’s growth rate would be decreasing which might be observable in the derivative of the derivative of acceleration? If that were happening, I wonder if virtual particles have some link to that process.

My reasoning here is that everything is accelerating from everything else since our position in the galaxy isn’t particularly special right? Then the only way for that really to be true would be for space time itself to be growing.

That's sort of what's happening. If you imagine space as a rubber sheet with the galaxies pinned to certain places, something is pulling the sheet apart, making it seem like the galaxies are accelerating away from each other. We know the math behind this mechanism thanks to general relativity (it only needs a certain energy density for empty space), but we don't know what causes it. So we just call it "dark energy." Without it, the original expansion of the universe caused by the big bang should be slowing down due to gravity. Dark energy causes it to accelerate instead.

Another amateur, but I think it's more likely the reverse. As we travel through time, us, and the atoms we are made of, shrink(or another way to look at it is 'being consumed', or using up its energy). And we shrink according to how fast we travel through time, and we move through time based on mass. The relative shrinking causes gravity. This would result in that same change in 'acceleration' that we observe, but really it's not that anything is moving, everything is simply changing sizes.

This must be wrong because it feels obvious and testable, simply make 2 satellites, send one to space, then confirm they are the same size when it gets back. And yet I can't find evidence either way.

>acceleration of acceleration

maybe this is where the c² in the famous equation comes from

>everything is accelerating from everything else

but what about galactic collisions? we are told the big bang did not happen from one single point but instead everywhere at once

That seems like it would itself need an explanation. Why would the initial conditions be objects starting at the ~same place but moving with random velocity?

The CMB would also need explanation and I'm thinking the curves of galaxy speeds wouldn't match unless you tweak the initial speeds way too much, but I'm not sure how to formulate that right.

I’ve always wondered whether there might be some other large scale effect happening that causes light frequencies to shift. What if photons travelling through a vacuum just slowly lose energy and their frequency drops? Would that be distinguishable from expansion by any of the measurements we can do here?

This is known as the “tired light” hypothesis, and there are a number of problems with it; see https://www.astro.ucla.edu/~wright/tiredlit.htm .

Oh cool, thanks! I’ve asked a few people about it but never received a satisfactory answer why it wasn’t a viable theory. This covers it nicely.

In line with your (possibly misguided) thinking, I too have some amateur thoughts that are probably equally wrong: https://pawel.fyi/post/red-shift

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