65 votes
Accepted

Shouldn't very very distant objects appear magnified?

Yes. And they are! This is called the "Angular diameter distance turnaround" (or turnover). In the usual model for expansion $\Lambda CDM$, it is at a redshift of about 1.5 or about 15 ...
James K's user avatar
  • 121k
30 votes

How far apart has the sun drifted from Alpha Centari due to the expansion of the universe since its formation?

The Sun was very likely nowhere near Alpha Cen when it was born. Stars born in the Galactic disc have velocity dispersions of about 10-20 km/s, which grow over time. A km/s velocity difference amounts ...
ProfRob's user avatar
  • 152k
28 votes

Is there physical evidence to distinguish between the expansion of space and an anthropocentric universe?

I'm looking for some kind of observable evidence (that has been observed, or could be observed in the future) that could falsify one or the other theory. You seem to be requiring one (very high) ...
StephenG - Help Ukraine's user avatar
23 votes

Are there any galaxies which fell out of sight horizon due to cosmic expansion?

No. In fact the opposite is the case. (See the last paragraph for an intuitive explanation.) It is a common misbelief that galaxies receding faster than the speed of light are not visible to us. ...
pela's user avatar
  • 38.2k
23 votes
Accepted

In km/h, what actually is the "speed" of Andromeda away from us: cosmologically?

The rate of expansion, measured in the customary units of (km/s)/Megaparsec is not known with great accuracy. Recent measurements include 67.6 (SDSS-III), 73(HST) 67.8 (Plank) 69.3 (WMAP) [wikipedia] ...
James K's user avatar
  • 121k
22 votes
Accepted

Will we start seeing galaxies disappear due to Universe expansion?

It is a common misconception that galaxies receding faster than light cannot be observed. There are two versions of this misconception: Galaxies that are now receding faster than light cannot be seen....
pela's user avatar
  • 38.2k
22 votes

How do we know the expansion of the universe is not centered around our position?

Taken from my answer to https://physics.stackexchange.com/a/534684/123208, which is a duplicate on Physics SE. We observe that galaxies appear to move away from us, in an isotropic fashion, at a rate ...
ProfRob's user avatar
  • 152k
21 votes
Accepted

What equation tells you how far in space you can go from a point and return?

Preparation Let $a(t)$ be the cosmic expansion factor. Let $x$ be a comoving coordinate, so two objects (like galaxies) that are separating solely due to the expansion of the universe have a constant ...
Sten's user avatar
  • 4,644
20 votes

How far apart has the sun drifted from Alpha Centari due to the expansion of the universe since its formation?

Cosmic expansion has no influence on how the distance between the Sun and Alpha Centauri evolves. Not even a negligibly small influence, but none whatsoever. The idea that cosmic expansion creates a ...
Sten's user avatar
  • 4,644
18 votes
Accepted

Could the redshift of all incoming photons be explained by a massive ring of distant masses pulling the sources of the photons away?

You have identified the issues. The model does not explain the redshift-distance relationship, which is one of the primary pieces of evidence. Simply to say "our ideas about gravity are wrong&...
ProfRob's user avatar
  • 152k
18 votes

Could we (Earth, Humanity, Solar System) be falling into a black hole?

No. There are two ways of understanding your question. One is "could there be a nearby, relatively small black hole (say, a few million times the mass of the sun) into which we are falling. No, ...
James K's user avatar
  • 121k
17 votes

How do we know we're not getting bigger?

Matter contraction: If everything got twice as big then you are right that rulers etc. would not be able to measure it. However, some physical constants are expressed (partly) in meters, and it would ...
Dast's user avatar
  • 271
17 votes

What equation tells you how far in space you can go from a point and return?

sten's answer is excellent and beautifully analytical; I just wanted to illustrate the journey and show that neglecting matter (and radiation) is a good approximation. Spacetime diagram To do this, I ...
pela's user avatar
  • 38.2k
17 votes

How do we work out the light travel time on a cosmic scale?

Let's first be clear that there is no unique way to identify the time or distance between two events. This is true in every relativistic context; just think about relativistic time dilation and length ...
Sten's user avatar
  • 4,644
15 votes

How do we know we're not getting bigger?

We do not know whether or not we live in a simulation in which our capricious simulation overlords have conspired to hide evidence that we are growing larger. Discounting that possibility, science ...
David Hammen's user avatar
14 votes
Accepted

How is the Universe's Expansion Accelerating if the Hubble Constant is Decreasing?

The Hubble parameter is defined as the rate of change of the distance between two points in the universe, divided by the distance between those two points. The Hubble parameter is getting smaller ...
ProfRob's user avatar
  • 152k
14 votes

Could there really be a preferential direction to the speed of light?

Prior to Einstein's 1905 paper, the Lorentz transformation had already been worked out by Lorentz and others. Only their interpretation of it was lacking. They still clung to the idea that there was a ...
benrg's user avatar
  • 3,852
13 votes
Accepted

How does the concept of a universe with no center work?

When we talk about the universe, we are really talking about one of two things: The observable universe, which is everything we can possibly see. The Universe, which is everything that has ever ...
Sir Cumference's user avatar
13 votes
Accepted

How do we know the expansion of the universe is not centered around our position?

Things are observed to recede with velocity $\vec v$ proportional to their separation $\vec x$ from us, $$\vec v=H\vec x,\tag{1}\label{1}$$ where $H$ is a constant (the Hubble rate). This relationship ...
Sten's user avatar
  • 4,644
12 votes
Accepted

Where does the energy of light go, when it red-shifts?

The problem is that conservation of energy is a slippery concept in General Relativity. There are arguments back and forth but most people accept that conservation of energy is only a local law - it ...
ProfRob's user avatar
  • 152k
12 votes
Accepted

Local neighbourhood and Superclusters

Structure formation Structure in the Universe — galaxies, galaxy groups, clusters, and superclusters — forms from regions of the Universe which are denser than the average; dense enough the overcome ...
pela's user avatar
  • 38.2k
12 votes

Does a measuring stick with a size of a billion lightyears in intergalactic space keep the same length in expanding space?

There is no Aristotelian/frictional force that drags things along with expanding space. The motion of objects is governed by inertia ordinary gravitation, which is an inward acceleration (note: ...
benrg's user avatar
  • 3,852
11 votes
Accepted

How far would EGSY8p7 be away now?

30.4 billion lightyears. The current distance — i.e. the distance that one would measure if we froze the Universe and started laying out measuring rods — is called the proper distance, or physical ...
pela's user avatar
  • 38.2k
11 votes
Accepted

If an object 1 billion light years away emits light, does it take more than 1 billion years to reach us because of the expansion of the universe?

Yes, in the time it takes light — or, in this case, gravitational waves (GWs) from the black hole merger event GW190521 — to travel from a source to an observer, the Universe expands, thus increasing ...
pela's user avatar
  • 38.2k
11 votes
Accepted

If we could fly off edge of observable universe what would happen?

The 'edge' of the observable universe is as much a edge as is the 'edge' of how far you can look from the roof of your house: none at all, it's just a limit to our vision. We can never reach this edge ...
planetmaker's user avatar
  • 19.4k
11 votes

Is there physical evidence to distinguish between the expansion of space and an anthropocentric universe?

The anthropocentric picture does not explain observations of the present and distant cosmic microwave background (CMB)? Are we to suppose there is a large, spherical shell of optically thick gas ...
ProfRob's user avatar
  • 152k
10 votes
Accepted

Are there other proofs of the expanding universe apart from the redshift?

Yes, there is direct, non-red-shift evidence of expansion. The past temperature of the Cosmic Microwave Background Radiation (CMBR) has been directly measured and found to be substantially higher ...
Alex Hajnal's user avatar
  • 1,189
10 votes
Accepted

Is the eventual heat death of the universe due to the expansion of the universe?

As far as I understand it, the heat death of the universe is a consequence of entropy, not expansion. All processes result in the shifting of some energy to higher entropy. Though the observable ...
called2voyage's user avatar
  • 6,312
10 votes
Accepted

Cosmological redshift vs doppler redshift

After considering @benrg's comments, I realize that my first answer contained too strong statements about the relation between the two redshifts. I try here to moderate my answer, but you might want ...
pela's user avatar
  • 38.2k
10 votes
Accepted

Why is the Cosmic Microwave Background evidence of a hotter, denser early Universe?

By request: Beyond the fact that the cosmic microwave background (CMB) is a direct prediction of the big bang model, there is the question of how you would produce it in any other way. It is ...
ProfRob's user avatar
  • 152k

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