# Tag Info

19

Basically, if two particles are placed with no other interaction between them, the distance between them will increase. Imagine living on the surface of a balloon which is being blown up. Your size stays fixed, because you're more or less rigid, but items not attached to you will move further away. Your ruler, another rigid body, stays fixed in size (though ...

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: ...

17

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. This is not the case; we easily see galaxies moving at superluminal velocities. This does not — as I think most people would think — contradict the theory of ...

13

The short answer is: gravitationally bound system will not be ripped by the accelerated expansion. A longer answer: The current standard model ($\Lambda$CDM model) says that everything started with the big bang. It released a lot of energy that pushed the Universe and is since then expanding. While it's expanding, gravitational attraction has been working ...

12

A good analogy which shows that the expansion has no centre is one of a 2D sheet covered with uniform dots - representing the galaxies where the one in the centre is us: As the universe expands the dots (galaxies) get further away from each other: Now if you overlay the future and present images lining up on the central galaxy (us) you can see that all the ...

12

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 existed, currently exists, and will exist. The observable universe has its own center, usually the Earth. It is a spherical region of everything that we can see, ...

12

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. If we observe a galaxy today, it may recede faster than light now, but when it emitted the light we see now, it did not. Both are incorrect. Intuitive ...

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

This is a tricky question to answer, because it has some false and imprecise assumptions baked into it. The short version is: I don't know at what rate stuff is currently crossing the cosmic Event Horizon. But future telescopes will not see these galaxies redshift themselves into oblivion, because this will happen in the infinitely far future, literally. ...

9

It would, if there were no interactions. However, everyday matter is held together by other kinds of forces (electromagnetic, mainly). These are quite strong, and any "stretching" of space will be counteracted by these forces "pulling" the atom/molecule/object back to its original size. So the expansion of the universe only matters on the larger, cosmic ...

9

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 distance, in astronomy. By definition, it corresponds today to another often-used term in astronomy, namely the comoving distance. While the former increases with ...

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

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 universe is an open system, the entire universe is an isolated system, so as more and more energy gets shifted to higher entropy over time the universe will ...

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

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

Yes, space is constantly being created. The new space does not hold any matter (like atoms) or dark matter. This means that the density of normal and dark matter decreases at the same rate as the volume increases. However, dark energy, which is something completely different and thought to be a property of vacuum itself, is being created with the new space, ...

8

The answer here is very similar to if you were asking about light. In principle gravitational waves might allow us to fractions of a second after the big bang. Electromagnetic waves can see back to where the cosmic background radiation formed, about 400,000 years after the big bang. You are right, the universe has expanded. At the present epoch it is ...

8

No - the decreasing energy in the CMB is already well modeled in the Friedmann equations. The term in the density parameter that is proportional to $a^{-4}$ is the contribution of radiation energy density to the evolution of the universe, the term proportional to $a^{-3}$ is matter density (mostly dark, but includes ordinary matter), $a^{-2}$ is the ...

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 The expansion of the universe it self does not cause atoms to increase in size (because of the electromagnetic forces holding them together), but it does cause galaxies to move apart from each other (e.g., observed galaxy redshifts and Hubble's Law). In reality, the expansion of the universe is known to be accelerating slightly (see Perlmutter et al., 1998)... 7 The solution to the Friedmann equation in a flat universe is $$H^2 = \frac{8\pi G}{3}\rho + \frac{\Lambda}{3},$$ where$\rho$is the matter density (including dark matter) and$\Lambda$is the cosmological constant. As the universe expands,$\rho$of course decreases, but$\Lambda$remains constant. Thus the Hubble "constant" actually decreases from its ... 7 Most$^\dagger$black holes are indeed expanding, but not because the Universe is expanding. Rather, their size (more precisely their Schwarzschild radius) increases proportionally to their mass, so they grow as they accrete more matter. It is a common misconception that everything expands along with the expanding Universe. It doesn't. On small scales, i.e. ... 7 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 because the denominator is getting bigger more quickly than the numerator. In the future, the cosmological constant,$\Lambda$could result in an exponential ... 7 Let me preface my answer by saying that you're reading a book from 1937 which references the scientific knowledge from the early 1900's. In case you're unfamiliar with astronomical history, I'll say that we've come a looong way since the early 1900's. For example, this book discuses "Nebula as Island Universes". At this point, astronomers had only just ... 7 The simplest explanation relies on the statistical unlikelihood of us being at a special place and time in the cosmos. Let's say, for a moment, that we see galaxies moving away from us because of some cosmic explosion that set them in motion. In that case, we would expect to see the galaxies in one direction moving faster than the galaxies in another. ... 7 A recent estimation of the Hubble constant is 71.9 (km/s)/Mpc. This means there is an expansion of $$\left(71.9\ \mathrm{\frac{km}{s\ Mpc}}\right)\left(\mathrm{\frac{60\ s}{min}}\right)\left(\mathrm{\frac{60\ min}{hr}}\right)\left(\mathrm{\frac{24\ hr}{day}}\right)\left(\mathrm{\frac{365\ day}{yr}}\right) = 2,267,438,400\ \mathrm{\frac{km}{yr\ Mpc}}$$ To ... 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

Expansion depends on the amount of mass Yes, space expands more in regions with less matter — in fact this has been proposed as an alternative explanation to dark energy as the cause of the observed accelerated expansion: If by chance we happen to be located near the center of a "cosmic void", then nearby space expands faster than distant space, and since ...

6

Is Cosmos seriously using that exact number? Egads... if they are, don't take it too seriously, but otherwise they're probably conceptually correct. How do we know it was dark energy? In cosmology, the ΛCDM model fits numerous observations, most notably those observed by the WMAP satellite, but also others, to the Friedmann-Robertson-Walker family of ...

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