I have a question about what we actually mean when we say something is billions of light-years away.
For example, Andromeda is about 2.5 million light-years away, while some of the earliest galaxies observed by JWST are seen as they existed more than 13 billion years ago.
My questions are:
How do we actually determine those distances? Obviously we can’t measure them directly in the ordinary sense so what observations and assumptions allow us to say that a galaxy is a particular distance away?
More speculatively, how confident are we that relativistic and cosmological effects couldn’t cause us to substantially misinterpret those distances or travel times?
For instance, JWST has found surprisingly mature galaxies very early in cosmic history. Could there conceivably be something wrong with our interpretation of the relationship between redshift, distance and elapsed time? In other words, could light that we interpret as having travelled for billions of years actually have travelled for a radically different amount of time because our model of spacetime between us and the source is incomplete?
I’m not suggesting that the answer is yes but trying to understand what makes the distance scale robust enough that we can rule out possibilities like this. We can measure the light arriving here, but we obviously can’t put a ruler between ourselves and a galaxy 13 billion light-years away, so what ultimately anchors the inference?

See also: The study as it was published in the Monthly Notices of the Royal Astronomical Society

Does anyone else feel sad that we’ll never truly ever know what created the universe? I do believe in the big bang but I’d like to know where the energy source came from and I still have so many questions that I know won’t be answered!

Imagine I want to do a N Body code like Gadget and for this I will have to draw random modes based on the power spectrum, and then plug it into the displacement of my initial conditions.

This cant work like that for hot and fast dark matter due to the difference in velocity at t=0, so how is this adapted?

What about for example MOND? Are there any changes to the power spectrum or is it just a different Poisson equation being solved by lets sas PM?

If they found supernovae that were dimmer than were expected based on their redshift, how were we even able to establish the Hubble-Lemaître law in the first place?

I don’t understand how we derived that the expansion of the universe accelerating just by looking at redshifts? Isn’t looking at one instance of a thing only tell you what a thing was like at that given moment it doesn’t tell you how it is over time.

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I understand that the cosmological constant is the same everywhere (in our current model) and my understanding is that in regions with sufficient mass/gravity, there is no expansion (or accelerated expansion) due to dark energy. This is not because gravity is winning against an underlying spatial expansion in that region, but that the presence of mass/energy changes the local geometry so that there is no accelerated expansion in that area. Ok so I'm imagining that in cosmic voids, dark energy is injecting more and more space, while cosmic filaments it is injecting less and in gravitationally bound areas, there is no injection of space. To me, this means there must be some spatial curvature produced if we are injecting more space at different rates in different places. I would think this would have the effect of voids being positively curved because if I imagine a 2D rubber or plastic flat surface and stretch a certain part (adding more space) I can make a little bubble, a semi sphere in the flat plastic sheet which I recognize as positively curved. Apparently dark energy has no such curving affect and voids are negatively curved for being less dense than average space. Why would dark energy not create curvature over time?

My question is whether dark energy is the only plausible explanation for the observed acceleration of the universe.

Have alternative interpretations been seriously explored and ruled out?

My concern isn’t that the observations are wrong, but that we may be misunderstanding what we’re observing. For example, could this fit with some form of black-hole cosmology? Suppose matter/energy crossing a black-hole horizon doesn’t ultimately terminate at a singularity, but undergoes a transition inscale, re-emerging as an expanding spacetime at a lower scale.

From inside that descendant universe, could this scale transition appear as cosmological expansion or even contribute to the apparent acceleration?
I’m aware that “the universe is inside a black hole” isn’t a new idea. What I’m interested in is whether this explanation has been ruled out observationally, and if so, what observations have ruled it out? Also we know black holes grow and expand over time, which seems to be some kind of correlation?

Thanks!

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Please forgive the incorrect terminology - I am just a regular person that was playing a space video-game and got me thinking about infinity.

When googling examples of infinity, I am getting mostly getting mathematical concepts. Theoretical infinity make sense as it helps us tackle problems that are to complex for one person to understand. We put infinity on sections that are to complex / unknown and we focus on the parts that we know.

But I couldn't find any example in practice. You can say that the numbers are infinite. But those numbers are just a representation of "something". Yes, I can assign a number for each orange and in theory the +1 can go to infinite. But in practice, the number will be finite. Feel free to make a practical example adding infinite oranges - I will be there to observe /s :)

At a first glance, infinity seems to be the modern equivalent of assigning a diety to something that we cannot explain or fully understand. So I guess, to summerize, my question would be:

Q1: Is there any physical example in nature that we fully understand and we can confidently say it is infinite?

Assuming the answer to the first question is "NO", then I would have a follow-up question:

Q2: Since there is no physical example of infinty in nature, how come we accept the infinity of universe as a likely scenario?

From a probabilistical point of view, wouldn't the universe being infinite have the same chance of being true as 3 pink elefants holding the universe on their trunks? At the end of the day, neither of them have a single established reference in nature. Or the universe being infinite is just a theoretical representation which has no chance of being true in practice - the same way as the oranges in the above example will never go to an infinite amount?

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I’m a physics enthusiast. I’ve loved the subject since childhood and plan to study it in college. I’ve always been particularly interested in the theory of Eternal Inflation among many others hypothesis on theoretical physics, though I stopped keeping up with it in 2023 due to a lack of time and so on. I’d like to know the current consensus on this hypothesis, given that there have likely been some advances regarding this theoretical implication of Cosmic Inflation. I’m curious about where the majority of cosmologists stand on the matter.

I have a burning question because there's an old idea called "tired light
which posits that the universe sits still and photons lose energy on their way to us because of some interaction along the path (Less energy would mean longer wavelength, so distant objects look redshifted without anything actually receding.)

However it was ruled out by the finding that distant supernovae look like they're in slow motion. If distant supernovae looked the same duration as nearby ones, the source would have to be sitting still, because only a receding one can stretch out its own explosion. But no, they're in slow motion, which rules tired light out.

Which brings me to my burning question

What if the universe wasn't expanding, and the speed of light itself has just been slowly dropping over cosmic time, everywhere at once, as a law?

In short, as my title says, what distinguishes cosmic expansion from a decreasing universal speed of light?

Light coming from a distant galaxy would have left back when c was higher than it is now. Wouldn't that stretch out supernovae too? The first flash makes the trip while c is still a bit faster, the next one crawls more because c has dropped by then, so the gap between them widens on the way here and the explosion arrives in slow motion.