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66 votes

How did Hubble know the red shift difference between "moving away" and "old"?

The "redshift" measurements that Hubble used to determine his law are based on looking at the spectrum of distant galaxies. That is, splitting the light coming from the object in its ...
TimRias's user avatar
  • 2,720
32 votes
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Does matter accumulate just outside the event horizon of a black hole?

Yes, you are absolutely right, from OUR VIEWPOINT it does. From Kip Thorne's book "Black Holes and Time Warps: Einstein's Outrageous Legacy." Like a rock dropped from a rooftop, the star’s ...
ctrebor's user avatar
  • 498
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
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
  • 123k
22 votes
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How do we know if an object is redshifted?

There are two methods, one more reliable than the other (though both are pretty good.) Key point: The brighter a star is, the more detail we can see in its spectrum -- you can think of it as being ...
Mark Olson's user avatar
  • 7,640
21 votes
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Can the gravitational redshift of our sun be measured?

Yes. It can be measured in spectra of the moon. A paper The solar gravitational redshift from HARPS-LFC Moon spectra describes the measurment of red-shifts in Iron absorption lines in the spectrum of ...
James K's user avatar
  • 123k
20 votes
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Which star / galaxy is moving away from us the fastest?

When a galaxy recedes from us, the light we see from it is redshifted. For galaxies at cosmological distances, this redshift is fundamentally different from a Doppler shift; whereas the latter is due ...
pela's user avatar
  • 38.6k
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
  • 153k
18 votes

How can redshifted light be detected?

In a redshift (whether that be caused by relative motion, gravitation or cosmological expansion), all wavelengths are increased by the same factor. Redshift is determined by identifying features in a ...
ProfRob's user avatar
  • 153k
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
  • 123k
17 votes
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Understanding The Turnover Point of Angular Diameter Distance

On the one hand an object spans a smaller angle the farther away it is, as expected. On the other hand, due to the expansion of the Universe and the finite speed of light, very distant objects were ...
pela's user avatar
  • 38.6k
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
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15 votes
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Can the difference between a star and a galaxy which are point sources be detected?

To distinguish galaxies from stars, you can use the spectrum. Roughly, stars have a black-body like spectrum with features depending on the absorption and emission on the line of sight and in the ...
cphyc's user avatar
  • 603
15 votes
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Is Webb or any near-future telescopes like ELT capable of observing redshift changes to confirm General Relativity?

The effect whereby, as the universe expands, the redshift of an object changes with time is known as redshift drift. A galaxy at a fixed co-moving distance will have a redshift that changes with time ...
ProfRob's user avatar
  • 153k
14 votes

Why do we see a cosmological redshift at all if space is not expanded in our solar system?

tl;dr Because space doesn't contract inside our Solar System. Wavelength increase is proportional to space expansion The prediction of general relativity — one of the most thoroughly tested and ...
pela's user avatar
  • 38.6k
14 votes
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Classifying 3C273 as a quasi-stellar object

When 3C273 was discovered then Hubble's law was well established - so if the redshift of 3C273 was non-cosmological then it would have to be (roughly) part of the local group of galaxies. However, ...
ProfRob's user avatar
  • 153k
13 votes

Can the difference between a star and a galaxy which are point sources be detected?

Even "round" galaxies look different from stars cphyc's answers the question excellently: Spectroscopy is the answer, although since — as explained below — galaxies are not point sources, ...
pela's user avatar
  • 38.6k
13 votes

What parameters of the Big Bang model will have to be adjusted to account for JWST's observations of highly redshifted galaxies?

The big-bang model does not have much to say about the redshift-dependence of the galaxy mass function (NB It is really the luminosity function that is being probed), other than that at some high ...
ProfRob's user avatar
  • 153k
12 votes

Can the difference between a star and a galaxy which are point sources be detected?

Good answers have already been given, but I wanted to provide another way of looking at it. Take a look at the image below, which is the Hubble Extreme Deep Field (XDF) $-$ for those who don't know, ...
zephyr's user avatar
  • 15k
12 votes
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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
  • 153k
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
  • 153k
10 votes
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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
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Where can I find a database of galactic spectra?

The Sloan Digital Sky Survey Data Release 15 contains over 4 million spectra of both galactic and extra-galactic origin from the multi-fiber spectrographs. Of these spectra, 0.7 million came from the ...
astrosnapper's user avatar
  • 8,357
10 votes
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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
  • 38.6k
10 votes

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

I think StephenG is right, but I will mention one counterfactual. Suppose we observed no galaxies more than a billion light-years away, as determined by their red shifts. Say they were roughly ...
Mark Foskey's user avatar
  • 3,916
10 votes

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

I would like to point out another flaw in the question: the model "everything moves away from us proportionally to its distance" is not actually anthropocentric in a basic approximation. Let'...
Magma's user avatar
  • 201
9 votes

Does matter accumulate just outside the event horizon of a black hole?

Several wonderful yet technical answers have been given, and I cannot add anything to those very nice answers that explain why it is not useful to think black holes get "frozen" at their event ...
Ken G's user avatar
  • 5,336
9 votes
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The "k-effect"?

The k-effect (or Trumpler effect) is an anomalous, additional redshift identified in the spectra of hot O/B and other luminous stars (Arp 1992). It is discussed in the article "Redshifts of ...
ProfRob's user avatar
  • 153k
8 votes

Could a contracting Universe create the redshift effect observed by Hubble?

Aside from being, I suspect, totally incompatible with GR, the model fails to match observations. The universe appears to be homogeneous and isotropic. In the model you suggest, Hubble's law would be ...
ProfRob's user avatar
  • 153k
8 votes

Does the accelerating expansion of the Universe contradict Hubble's law?

The Hubble law gives the velocity of a distant galaxy right now. A galaxy at a distance $d$ recedes at a velocity $v = H_0\,d$ right now$^\dagger$. However, the relation between $d$ and the redshift —...
pela's user avatar
  • 38.6k

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