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# Tag Info

25

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 distant and stationary relative to the black hole. You will observe infalling matter asymptotically approaching the horizon, growing ever fainter as it redshifts. ...

24

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 falls downward (shrinks inward) slowly at first, then more and more rapidly. Had Newton’s laws of gravity been correct, this acceleration of the implosion would ...

22

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 able to magnify the spectrum more so as to be able to see finer details. This also allows us to see fainter lines (not all spectral lines are equally intense.) ...

19

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] The Andromeda galaxy is 0.78 Mpc from us, so taking the Hubble constant to be about 70, gives a recession of about 55 km/s. This is not a very great speed: ...

18

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 to a velocity difference between the emitter and the receiver, a cosmological redshift is due to photons traveling through an expanding space$^\dagger$. Hence, ...

16

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 being violet light, with a 668–789 THz frequency. Red Shift is an observable effect when you analyze the spectroscopic signature of an electromagnetic radiation ...

15

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 indeed bluer than the red galaxies, but there is no absolute scale with which to judge redshift by eye. Secondly, there is no detail in the NASA web page, but the ...

14

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 chromosphere of the star. Galaxies on the other hand of a spectrum that is the composite of tons of stars. The spectrum will for example be much wider (ranging ...

13

Stellar Parallax Stellar parallax uses differences in perspective to determine the distance from an object. When the earth goes around the sun, our perspective of the star, galaxy etc. changes and so the angle from us to the object changes. Because we know how the earth moves around the sun, we know the distance between the points that we take the ...

13

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, the morphology of stars and galaxies is also different: even elliptical galaxies observed along one of their axes look different from stars. Although both are round, the ...

12

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, this is a small patch of the sky that Hubble has stared at for a total of 23 days over 10 years $-$ and you'll notice something interesting. It is clear to see ...

11

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 come to terms with just what is happening as opposed to what appears to be happening. The experience of time We must remember that an object moving at a certain ...

11

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 applies only to a local inertial frame and cannot be applied to the universe as a whole. However in an expanding universe it is very difficult to identify any ...

10

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 original spectrographs during the SDSS-I/II Legacy Survey and the remainder from the upgraded spectrographs as part of the BOSS survey during SDSS-III (see SDSS ...

9

Looks like you can measure the cosmological redshift of quasars using that equipment and an 14" reflector: http://www.rspec-astro.com/sample-projects/ (halfway down the page) So the answer appears to be yes, you can do it, assuming that page isn't a total fabrication. Seems plausible to me: the redshift of that quasar is sizable at 0.15 (though that's ...

9

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 glowing from being heated.If you looked at one of those orange colored sodium vapor street lamps, instead of a smear of color, you'd see a group of lines. This ...

9

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 than it is today. Its reduction in temperature over time is direct evidence of expansion. Here are the details: According to this paper, the CMBR was measurably ...

9

I think your confusion has to do with terms and semantics, rather than physics: The cosmological redshift has nothing to do with the velocity of the emitter and the observer with respect to each other. That's why it's not a Doppler shift. The cosmological redshift is caused by the expansion of space. It is a direct measure of the relative size of the ...

8

Reason 1: Let's look at the Friedmann equations without the cosmological constant. $$\frac{\dot{a}^2 }{a^2} = \frac{8 \pi G \rho}{3}-\frac{kc^2}{a^2}$$ The term on the LHS is just the Hubble constant squared $H^2$ which can be measured the direct measurement of recession velocity of galaxies The density term can be said to be a combination of $\rho_{... 8 I will make a small calculation here, but please proceed to the results if you may like to. Calculation Stars are spherical and static, so metric near their surface (photosphere) and outside on is Schwarzschild. Hence time-time metric component on the surface is: $$g_{44}=1-\dfrac{R_{grav,*}}{R_*}$$, where$R_*$is the radius of the star and$R_{grav,*}$... 8 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 and you can spot the pattern of those colours when the whole pattern has been shifted towards the red and see how far it has shifted. For closer stars, there ... 8 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 recede at$v=H_0 \,d\simeq6100 \,\mathrm{km}\,\mathrm{s}^{-1}$. However, galaxies also move through space, at typical velocities from several$100\,\mathrm{km}\,\...

7

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 galaxies detected now have redshifts of 7 or more. This means the wavelength of their light has been stretched by a factor $1+z$ - i.e. by a factor of 8. Thus light in ...

7

Stars consist of similar atoms and molecules that we can find here on Earth. Radiation from those atoms and molecules often occurs at discrete, identifiable wavelengths that form a unique pattern. This is a consequence of quantum physics - the energies of atomic and molecular states are quantised, and radiation arises from transitions between these. These ...

7

Redshift is the "stretching" of the wavelength of light as it travels, over finite time, towards us, due to the ongoing expansion of space. The effects of this redshift are indistinguishable from that caused by the doppler effect, so that is why some elementary treatments choose to discuss it in that way. Most galaxies only have peculiar velocities with ...

7

Yes. This is is actually something that is done in the analysis of redshift data. If no correction to the redshifts are made, then the redshifts are known as "geocentric." When you correct for the motion of the Earth around the Sun, the redshifts are known as "heliocentric" (see, for example, the description of data in the 6dFGS database). Less common is ...

7

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 — which is the quantity that we observe — is a non-trivial function of the expansion history of the Universe, obtained by integrating the (inverse) scale factor ...

7

But is it still possible that the redshift is caused by some unknown phenomena and not by galaxies moving away from each other? In history some alternative theories were proposed, like the tired light hypothesis, the steady state universe etc. But the observation ruled these and other theories out. See also Alternative cosmology

7

Photons are massless. This doesn't depend on their energy, so doesn't depend on their frequency or wavelength. Massless particles travel at the speed of light. Even if we abandon particles and look at classical electrodynamics, we find that the speed of an electromagnetic wave (in vacuum) has a fixed value. It doesn't depend on wavelength. Gravitational ...

7

The optical emission lines of quasars do not come from sufficiently close the the central supermassive black hole to be appreciably gravitationally redshifted. If they did arise from gas near the "innermost stable circular orbit", then the maximum gravitational redshift would be about 0.2. In addition, the lines would have a characteristic profile caused by ...

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