22

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 ...


20

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: ...


20

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 ...


13

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 Newtonian absolute time, and the times in the Lorentz transform were only apparent times. Einstein was the first to realize that there doesn't need to be a ...


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

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 the expansion of the Universe and collapse gravitationally. Whereas it was previously thought that the largest structures collapsed first, later fragmenting to ...


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 ...


11

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 though for the observable universe, as the limit to our movement is the speed of light - and the edge recedes faster than the speed of light. There is no ...


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. ...


10

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 ...


10

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 the distance further. Various distance terms In the following, "$\mathrm{Glyr}$" means a distance of a billion lightyears, while "$\mathrm{Gyr}$&...


9

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, ...


9

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 ...


9

This is an intriguing proposition, but I would ask how your hypothesis explains that the universe appears to be flat? That is with $\Omega_M + \Omega_\Lambda = 1$. The evidence for this comes from measurements of the cosmic microwave background, yet if we sum up all the matter (including dark matter), we only arrive at $\Omega_M \sim 0.3$. I do not think ...


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

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 to accept their answer instead. It is common to think of the two redshifts as having nothing to do with each other. Doppler shifts arise when the observer and/...


9

The age of the universe is not calculated based on the size of the visible universe. The age of the universe is being calculated based on the fact that the laws of nature have no direction. This means that you can use the laws of nature to predict future behavior, but also assume previous behavior. Based on calculating backwards with the laws of nature, for ...


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

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

As time passes, there are galaxies that are currently not in the observable universe which will become observable But this is not a sudden winking on. Instead, over hundreds of millions of years we will see a proto galaxy evolve into a mature galaxy. For example there is a "blob" of hydrogen that some interpret as being the accretion of hydrogen onto a ...


8

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 ...


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

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 ...


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}\,\...


8

There is no locally measurable force associated with the Hubble expansion. The short reason is in ProfRob's comment: "the little piece of the universe you are considering doesn't obey the approximations (being homogeneous and isotropic) that lead to the Hubble expansion." For more details, see this answer. In ΛCDM cosmology, $Λ$ does cause an ...


7

Conceptually there are several things going on here. Where does energy conservation come from? In modern understanding, energy is the Noether charge of time translation symmetry, as found by Noether's first theorem. But in general relativity, the metric is dynamical, and so in general we simply don't have any time translation symmetry. Static spacetimes do,...


7

Expansion means that distances are increasing as a function of time. Say if the distance between two galactic clusters is $D$, then in an expanding Universe the distance is governed by some strictly increasing function of time $a(t)$ called the scale factor where $$D=a(t)D_0$$ where $D_0$ is the distance at the present time and by definition $a(t_{0})=1$. ...


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

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 ...


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