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 ...
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  • 1,942
30 votes

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

What you're describing is basically the "collapsed star" (Eng) or "frozen star" (Rus) interpretation of black holes that was common prior to the late mid-1960s. It was a mistake. Suppose you are ...
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  • 7,674
29 votes
Accepted

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 surface ...
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  • 462
25 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) ...
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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 ...
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  • 7,390
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 ...
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  • 88.7k
20 votes
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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] ...
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  • 88.7k
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 ...
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  • 32.3k
17 votes

Why are there so many seemingly blue-shifted galaxies in deep space

You cannot gauge the redshift of a galaxy by looking at a false colour image. The images taken through different filters are stacked and colourised to suit. You can say that the blue galaxies are ...
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  • 115k
16 votes

How Do we know about redshift?

Let's start with a quick clarification: Red Shift is not the same as red light. Red light is just electromagnetic radiation with a 400–484 THz frequency range, the lowest our eyes can see - highest ...
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  • 715
14 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 ...
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  • 593
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 ...
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  • 32.3k
14 votes
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Declaring of 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, ...
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  • 115k
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, ...
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  • 32.3k
12 votes

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

We need to think about just where the time dilation effect occurs. By then thinking about the observations from each point of view, that is the free falling object and the external observer, we can ...
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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, ...
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  • 14.4k
11 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 ...
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  • 115k
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 ...
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  • 7,170
9 votes
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How do we know that light is redshifted/blueshifted and not the original light of a star/galaxy?

If you had a simple slit spectroscope, and looked at an incandescent light, you'd see a smear of light with red on one end and blue on the other. This is because the filament is producing light by ...
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9 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 ...
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  • 1,171
9 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 ...
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  • 32.3k
9 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 ...
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  • 2,525
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 ...
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  • 115k
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 —...
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  • 32.3k
8 votes
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How do we measure distance in space?

Red shift is used for measuring the distance to very distant stars (galaxies mostly, in fact). The secret is to use spectral lines. Specific elements when very hot emit light at very specific colours ...
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  • 9,893
8 votes
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Why does the Supernova 2006cm give a very different value for the Hubble constant? Why doesn't it increase error bars for the Hubble constant?

At a distance of $d = 87\,\mathrm{Mpc}$, with a Hubble constant of roughly $H_0 = 70\,\mathrm{km}\,\mathrm{s}^{-1}\,\mathrm{Mpc}^{-1}$ cosmological expansion should make the host galaxy UGC 11723 ...
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  • 32.3k
8 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'...
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  • 181
8 votes

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

I'm not clear what, in this anthropocentric picture, explains the cosmic microwave background? Are we to suppose there is a large, spherical shell of optically thick gas expanding away from us faster ...
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  • 115k
7 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 ...
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  • 5,142
7 votes
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Have we detected galaxies which have red-shifted beyond the visible light range?

Yes, of course. Many, many examples. Telescopes work in the infrared, far-infrared and there are even samples of galaxies that are selected on the basis of their mm emission. The most distant ...
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