# Tagged Questions

Space expansion is a cosmological phenomenon wherein the proper distance between two spatial points for a given inertial reference frame increases from one moment of time to another. That is, space itself expands; the added distance is not due to relative motion of points or objects.

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### In what manner does momentum of a particle with mass decrease due to spatial expansion?

I've read that the momentum of particles declines due to the universe's expansion. In particular, that $p \propto \frac{1}{a}$, where $a$ is the scale factor. For light, this momentum reduction ...
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### Where does the kinetic energy come from if galaxys are moving apart due to the expansion of the universe?

We know the universe is expanding because of the red shift of far away galaxies, the further away the galaxies are the faster they recede. We also know that the energy in the universe (mass and other ...
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### Expanding Universe Balloon Analogy - Anything Similar for Time?

It is difficult to imagine the infiniteness of space and how it itself is expanding rather than the universe expanding into something else. A helpful analogy is that of drawing little dots (...
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### Does the FRW metric imply spacetime scales?

The FRW metric can be written in conformal co-ordinates to give: $$ds^2=a^2(\eta)(-d\eta^2+d\mathbf{\Sigma^2}),$$ where $\eta$ is the conformal time and $\mathbf{\Sigma}$ ranges over 3-dimensional ...
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### How is the speed in Hubble's law determined, if not from the Doppler shift?

Hubble's law originally mentions the speed (i.e. proper motion) of a receding galaxy, as calculated via the Doppler effect from the observed redshift. However, the observed redshift is today explained ...
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### Is our locally measured time actually conformal time?

The FRW metric at the origin $r=0$, with $c=1$, is given by: $$ds^2=-dt^2+a(t)^2dr^2$$ Now one can change variables so that near the origin the FRW metric is approximated by the Minkowski metric ...
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### At what expansion rate (H) would virtual particles be ripped apart into real particles, and what might be the density (temperature) of such an event?

Is it even possible for a virtual particle-antiparticle pair to be torn apart by cosmological expansion? Virtual particle-antiparticle pairs are ripped apart near black hole event horizons, creating ...
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### When spacetime expands to the point where galaxy clusters are not observable, will there by any interaction?

It's my understanding that in a few billion years, clusters of galaxies won't be able to directly observe one another due to the expansion of spacetime overcoming gravity between those clusters. ...
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### Dark energy and gravitation force

This might be a silly question to ask but still I had to ask. we know that gravitational force between two galaxies are quite less as compared to the dark energy. so two galaxy are separated at speed ...
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### The Doppler shift and the apparent speed of galactic rotation with distance

A Doppler redshift would also give the illusion that galaxies were rotating more slowly then they are with the degree of illusory slowing in proportion to the degree of redshift. Do more distant ...
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### Does cosmological time expand like space does?

The FRW metric is given by: $$ds^2=-dt^2+a^2(t)\ dr^2$$ where $ds$ is an interval of proper length, $dt$ is an interval of cosmic time, $dr$ is an interval of co-moving co-ordinate distance and $a(t)$ ...
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### Metric to describe an expanding spacetime from coordinates reflecting the perspective of a local observer

The FLRW metric describes the metric expansion of spacetime from the perspective of comoving coordinates. Given the way this metric is usually formulated, comoving distances stay constant, and the ...
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### How fast is a true vacuum bubble going to expand?

If a true vacuum bubble will occur in our universe, how fast is it going to expand?
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### Cosmic Inflation: Lower Expansion Rate with than without?

When I read the Cosmic Inflation diagram at https://en.wikipedia.org/wiki/File:Horizonte_inflacionario.svg which I rotated to make it readable: where the Hubbleradius, and therefore also the ...
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### Can decompactification explain the inflation of the early universe?

I've just reread chapter 11 of this book where it is explained among other things, that our four dimensional universe could be unstable concerning a decompactification transition, since potential ...
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### composition of space expansion and movement as a gauge invariance

suppose i have a space-time where we have one point-like object* which we will call movement space probe or $\mathbf{M}_{A}$ for short, and it will be moving with constant velocity $V^A_{\mu}$ in ...
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### The expansion of the universe and its edge

I have understood that the universe is expanding at an accelerated rate. I have also thought, perhaps mistakenly, that this expansion means not that the masses in the universe are moving away from ...
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### The reason for the fading of galaxies?

Galaxies that are greater than a distance of c/H metres from Earth have recessional velocities exceeding the speed of light and begin to fade. Thus, theoretical astronomers 3 trillion years the future ...
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### The steady state theory, can it now be falsified on particle physics grounds, in addition to CMB data?

The steady state theory is no longer taken seriously by most physicists and the Big Bang theory is supported by an enormous amount of evidence, especially the CMB data. But from a quick scan through ...
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### Significance of light travel time in cosmology

When in astronomical news they say something like "It took $T$ billion years for the light from that galaxy to reach the Earth" or "The light traveled $L$ light years" what is the physical ...
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### Instabilities in the CDMT

Could anyone explain or refer to references on why the CDMT f(R) gravity model suffers from Instabilities any why the sign of ${\mu}^{4}$ matters.
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### Is the Hubble parameter affected by 'peculiar velocity'?

Are there any direct observations showing the effect of peculiar velocity on the Hubble parameter. If the Hubble parameter is measured in the direction of our motion caused by our rotation in the ...
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### How often can virtual particle/antiparticle pairs form real particles in a space expanding sufficiently fast to do this?

And if the rate of expansion of the universe becomes sufficient for virtual particles to be ripped apart into real particles, what effect would the mass of these particles have on the rate of ...
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### Intuition behind deriving the FRW metric

I am studying the FRW metric and am looking at a motivation for it here. The motivation seems to use four spatial dimensions. Why do we need the fourth spatial dimension in this? This doesn't seem ...
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### What is the relationship between an expanding universe, an increase in entropy and the the creation of mass?

What is the relationship between an expanding universe, an increase in entropy and the the creation of mass? Could the creation of mass be driven by or in equilibrium with the creation of space?
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### Scale factor of the universe

Do we impose that the scale factor $a(t)$ of the Universe is a continuous function? Or there is a physical meaning? Usually in physics we define functions to be continuous, such as the velocity of a ...
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### Is it accurate to say that nothing can travel faster than $c$ in a GR context, where more space can be created?

Years ago, my brother and I had an argument where I was trying to convince him that nothing could travel faster than the speed of light. I was pursuing this in the context of Special Relativity. My ...
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### Does conservation of moment hold in space expansion?

I recently watched a video describing that the expansion of space is a "stretch". Everywhere in the universe expanding together. My question is, does conservation of momentum hold true in this ...
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### What evidence is there of a universe older than 13.8 billion years

I've read an analogy that finding iron-rich galaxies just 900 years after the Big Bang is like finding an old man in a crib in a nursery. We just recently found a supermassive black hole 12 billion ...
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### Assuming the universe is finite, at some point, CMB should go away, right?

I'm not talking about gradual red-shifting, that happens slowly anyway, but with a universe of finite size, we should actually see Cosmic Background Radiation turn off in a sense, first at a point, ...
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### Center of mass & Big Bang Theory

I have read in my text books about the Big Bang theory and quote "the entire matter in the universe was once concentrated in a single very dense and hot fire ball; it exploded some 20 billion years ...
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### extreme heat to extreme cold (define the endstate)

Contemporary cosmology frequently has space temperatures just after the 'big bang' in the regions of millions of degrees and with inflation and expansion of the universe this is now down to a couple ...
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### Inflation of universe and Information stored therein

In an inflationary model of our universe, the space-time expands within a finite boundary. Some other models are also proposed where the universe is not explained to have been expanding rather the ...
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### What's the timescale until the Medium Crunch?

The future of the universe seems to be: clusters of galaxies drift apart due to space expansion until they all pass out of each other's cosmic event horizon. Gravitationally bound clusters overcome ...