Questions tagged [space-expansion]

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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How has the size of the universe changed as a function of time? [duplicate]

Is there a plot or an equation that calculates/shows the size of the universe for each era? Preferably starting from the Big Bang, inflation and on up to today.
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SFR Breeders on Small Mars Moon feasibility

Need help with a topic for a sci-fi book. Basic premise is 200yr in future, mining facility on small Mars moon of Phobos. Power source we would like is an SFR breeder as we think this would be cost ...
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Does the homogeneity of space imply that the expansion of the universe is uniform?

Obviously, homogeneity implies that the density is the same everywhere at any time. However, does this imply that the expansion is uniform? By uniformity, I mean that if I pick three galaxies to form ...
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Runge-Kutta fourth order method. Integrating backwards [on hold]

I am using a Runge-Kutta fourth order method to solve numerically the usual equation of motion of a background scalar field in curved spacetime with a quartic potential: $\phi^{''}=-3\left(1+\frac{H^{...
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Conservation of Charge vs. Conservation of Energy

Is conservation of charge ever violated like conservation of energy is violated during cosmological expansion? I am trying to understand this with respect to Noether's Theorem.
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What happened to a photon in the early universe?

So in the early universe where the universe was really small what would happen to a photon is it didn't collide with any material and made it to the edge of the universe? Because yes the big bang ...
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An “upper ceiling” for thermodynamics?

Roger Penrose said in "A Road to Reality" (p.701): “There is a common view that the entropy increase in the second law is somehow just a necessary consequence of the expansion of the universe. ...
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Using Hubble parameter to compare sizes now and then

What if the Hubble parameter is constant both in time and space. How would the scale factor look like? For $H_0=70~kms^{-1}Mpc^{-1}$, how much smaller would the universe be 14 billion years ago? I'm ...
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Did the big bang start at ONE infinitely curved point (or a 4D curved ball shrunk to a point) with an infinite density of energy?

I'm confused. In this question it is said: The simple answer is that no, the Big Bang did not happen at a point. Instead, it happened everywhere in the universe at the same time. How could it have ...
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What is the expected size of primordial black holes?

I often see it implied that primordial black holes tend to be small compared to stellar masses; Since primordial black holes did not form from stellar gravitational collapse, their masses can be ...
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Different versions of the Robertson-Walker Metric

One form of the Robertson-Walker metric is $$ds^2 = c^2dt^2 - a(t)^2[d\chi^2+ S_k(\chi)^2(d\theta^2 + \sin^2\theta ~d\phi^2)]\tag{1}$$ $$\\$$ Considering curvature, where k = 0 , +1, -1 for flat, ...
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What if there were only 1 generation of fermions?

The number of fermion generations has many effects beyond the mere existence of "3 copies of fermions," including determining scatterings, constraining CP violation, influencing electroweak symmetry ...
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Changes to a Length of Physical Ruler Caused by Gravity vs Caused by Cosmological Expansion of Space

I read here (Feynmann Lectures, Lecture 42) that "Just as time scales change from place to place in a gravitational field, so do also the length scales. Rulers change lengths as you move around." (...
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Why is light, that was emitted right at the big bang (same time/place), arriving to us at different times [duplicate]

I understand the big bang to have (at least mathematically) started our universe at a virtually precise moment and within a virtually infinitesimal volume. Yet some of the light from that moment/...
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If A Very Distant Galaxy Has Never Been Observable By Us, Then It Never Will Be? [duplicate]

I understand that distant galaxies that are currently inside our particle horizon will, sometime in the future, not be observable by us anymore because they are receding faster than the particle ...
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Measuring Speed of Light with A Ruler in Expanding Space

At $t_0$, we have a ruler which has two tick marks (one at each end) with one labeled "$0$" and the other labeled "$L$" (it is $L$ units long). Suppose at that time we also have two point objects ...
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Quantum tunneling of dark energy from black holes

There was an article posted recently suggesting there is a very deep connection between dark energy and black holes. https://phys.org/news/2019-09-black-holes-dark-energy.html I was thinking this ...
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Increasing expansion rate and time dilation

Does the increasing dispersion of matter that occurs as the universe expands impact time (and therefore observed speed) for older/further compared to newer/closer events being observed simultaneously ...
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What is the density of a static dust sphere in an accelerating universe?

Consider a largish but finite sphere of pressure-less homogeneous dust late in the $\Lambda$CDM cosmology. The outside metric is presumably Schwarzschild-de Sitter with no matter, the interior a ...
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Is every observer equidistant in all directions from the point of origin of the universe? [duplicate]

I am interested in the origin and present structures of the dimensions of space and time. I do not think space/time can be correctly described by an ordered set of scalars: (x,y,z,t). So. If every ...
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Does cosmological expansion change the gravitational field around a massive body?

Do expanding universe affect the curvature of spacetime? If so, does Einstein's field equation account for the change in curvature (gravitational field) around massive object in the expanding universe?...
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Why did dark energy play a subdominant role in the radiation and matter dominated era?

After inflation, the Universe passed through radiation dominated (RD) and matter dominated (MD) era during both of which the evolution of the scale factor (or the Hubble parameter) was decreasing with ...
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Why Cosmic Microwave Background is still present? [duplicate]

This is a novice question. It is theorized that Cosmic Microwave Background (CMB) is remnant of radiation from very early time of universe creation. As these were photons that have been traveling, ...
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Gravity as a repulsive force

As we know that universe is expanding, why can't we think the gravitational force as repulsive in nature in the realms of dark matter and dark energy?
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“Cosmological constant problem” and dark energy

Simply stated: is the "cosmological constant problem" (the discrepancy of about 120 orders of magnitude between measured values of vacuum energy and the ones predicted by Quantum Field Theory) in any ...
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What is a layman explanation for why the wavelength of light increases when space itself expands?

I understand that the universe is generally agreed to be expanding based on observations. What I have read is that space itself is expanding. My question is why does this expansion of space affect the ...
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If light speed determines the rate of time, could the speed of light be varying over time and we be unaware of it?

Is this a valid implication/alternative explanation of universal red-shift? I thought I'd ask this when I read this question about light clocks. I had speculated this way: If light speed determines ...
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The limit of the Universe that we'll ever be able to see

After watching Neil Turok's talk at 13:00 - 13:20 he says that the Universe expands and there is a limit beyond which we will never be able to see anything. And that's due to the Dark Matter which (...
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Expansion of the universe – impossible for future astronauts to make it back

The universe is rapidly expanding. If future astronauts built a spaceship that could travel close to the speed of light and took off, there is a certain point where they could not make it back again ...
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Does the age of the universe depend on the way it is defined?

I was reading some earlier posts on this question in stack exchange. These posts give me the impression that the age of the universe is defined in a certain way following prescriptions such as that: ...
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Is the observable universe equivalent to 'our' light cone?

All the objects we can observe (stars, galaxies, ...) must be in our past light cone, since otherwise we couldn't see them. Presumably there are more objects located outside of our observable universe ...
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If inflation is an exponential expansion, is the exponent known?

Inflation is often called exponential expansion. I can't seem to find what that exponent is. Is it 2, or 20, or completely unknown?
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'My question is about' …the size of the Universe after Inflation [duplicate]

Hitherto, I had assumed, on account of Inflation, "boom" the Universe was "out there" and it has taken light 13.6 BY to get here. However... Quoting the aerospace engineer: In my thinking if the ...
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Why physics cannot affect cosmological modes whose wavelength is larger than the horizon?

I started to learn cosmology recently. Confused about this point in a book. The book is Modern Cosmology by Scott Dodelson. I am confused about this in Chapter 6 "The horizon grows as the scale ...
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Increasing the view of observable universe

I have read that the maximum observable universe is about 92 billion light years in diameter by measuring the microwave background waves emitted after some few million years ago from the big bang and ...
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Forces between adjacent particles during big crunch

Ignoring the effects of gravity, consider two adjacent objects/particles in contact. If the universe was expanding, this would cause them to separate at $\vec{v}=H_0 \vec{r}$, where $H_0$ is the ...
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Is the “cosmological constant tension” the prime reason that we believe the expansion of the universe is accelerating?

Background: For a few years now there is significant difference in the calculation of the cosmological constant between standard candles methods and CMB methods. CMB measurement comes from a very ...
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If the universe expands does spacetime,dark energy,dark matter,gravity expands also?

If the universe expands does spacetime,dark energy,dark matter,gravity expands also ? Do forces,energy/matter expand with the universe also ? When we say that the universe is expanding ,what is ...
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Does space expansion affect the CMB photon frequency?

I have read this question: Effect of expansion of space on CMB where Ted Bunn says: For definiteness, let's consider a wave packet of electromagnetic radiation with some fairly well-defined ...
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How can I compute the scale factor for an open universe?

I'm struggling in obtaining an analytical expression for an open-universe $a(t)$. I know it is usually calculated from the second Friedmann equation, and with respect to the image I know how to ...
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Intergalactic medium and cosmic expansion

Presumably, the intergalactic medium (IGM) should get less dense as the universe expands. Also presumably, There should be a Hubble redshift to absorption lines due to the IGM. Could this provide ...
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Does $G$ effectively change in an expanding metric?

Assume that we have a flat FRW metric expressed in conformal time $\eta$ so that the line element is $$ds^2=a^2(\eta)(d\eta^2-dx^2-dy^2-dz^2)\tag{1}$$ where $a=1$ at the present time $\eta=0$ and the ...
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What's the advantage of Conformal Time?

I'm trying to follow the analysis of CMB Acoustics and several charts are done using conformal time instead of chronological time. Conformal time corresponds to the quotient of the particle horizon ...
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Why does Penrose expect visible Hawking Points in CMB?

I've only just started reading into Conformal Cyclic Cosmology. My understanding of the hypothesis is that, with some conformal transformation of space, the very end of the universe at $t=\infty$ ...
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Could decaying dark energy solve the Hubble tension? [closed]

The redshift-based distance ladder values of the Hubble constant are higher than that inferred by the cosmic microwave background. This discrepancy in the last few years has become highly ...
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Is the energy density of vacuum uniform in the whole universe?

This is not a duplicate. I am not asking why the energy density of quantum vacuum stays constant while the universe expands. My question is about how we know/measure that this vacuum energy density is ...
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Is it possible that the universe is not expanding faster than the speed of light? [closed]

What if the speed of light increases proportionately to the expansion of space? Is it possible that light traveling in the medium of space gets to travel faster if the medium is expanding? An analogy ...
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Why is the measure problem an issue if the universe is always finite?

As a follow-up from this question, I was wondering how the measure problem applies to the idea of eternal inflation. First, why is it even a problem? As time progresses, more universes are created in ...
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How necessary is inflation given the laws of physics?

How deeply is cosmic inflation required for the laws of physics? Is inflation required for a universe remotely like ours, or is it simply a contingent on the starting conditions of the universe? For ...
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Inflation v.s. Big Bang

It is said that that transition or certain quantum rotor field, and “rolling” down into the valley (of some quantum potential), is what causes inflation to come to an end, and create the hot Big Bang. ...