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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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. ...
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COSMIC ACCELERATION

Please forgive a probably naïve question from a lay person. I understand that galaxies are all moving away from us in every direction we look. And for any given distance the speed is the same. And ...
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If time slows in the presents of gravity, how does the conclusion that the universe’s expansion is accelerating account for this?

I am sure there is a simple explanation for this and I’m not a physicist so hoping someone here can help answer this for me. Time is retarded by gravitational field. Expansion in all directions ...
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Is the expansion of the universe uniform? Are there parts which are expanding faster than others?

Is the expansion of the universe uniform? Are there parts which are expanding faster than others? The very fabric of the universe is expanding..but is it expanding at a constant rate? If it is not ...
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Does curvature affect the change in the acceleration/deceleration of the Universe?

Suppose I'm modelling a Universe with non-zero curvature, filled with matter, radiation and dark energy (further described by quintessence). The appropriate Friedmann equation would be of the form: $$...
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Could the matter for a Big Bang come from the Hawking radiation of a super fast expanding universe?

If I understand it right, the universe will at some point in time expand so fast, that from any particular point in space, an observer will see an event horizon around himself. This event horizon ...
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Why does the expansion of space lead to cosmic redshift?

I have always thought of light as a point-like object in space, with the EM field strengths dependent on time. However, stretching of space would not affect a point-like object. Thus, I assume that ...
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Is the expansion and contraction of the Universe an endless repeating cycle due to Newton's 3rd Law of Motion? [closed]

If the creation & expansion of the Universe was an action, then the reaction to this action is that the Universe will contract and be condensed back into a singularity. If the contraction of the ...
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How does the cosmological constant dominate cold matter perturbations?

I have been studying inhomogeneous universe in physical cosmology, whereupon I arrive at cold matter perturbations which are dominated by various background fluids. I understood the solutions for the ...
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How do we determine both the distance to and speed of distant stars?

My understanding is that we can measure the distance to far away stars by using their color to infer the absolute magnitude of the light they emit and compare that to the observed magnitude of the ...
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Does CMB rule out that the universe is infinite?

If the universe were infinite, the energy of the big bang would have been long dissipated, and very little or nothing would hit us. Does the fact that CMB still comes roughly the same from every ...
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Can we calculate the expansion rate of the Universe at each moment in time?

Do we know the expansion rate of the Universe at each moment of time $t$ of its history? We only measured the expansion rate for a few decades (let's say), which isn't much compared to the age of ...
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How is observable universe so big if the universe is so young? [duplicate]

The diameter of observable universe is 93 billion light years but the age of universe is only estimated to be 14 billion years. So how does light have 46.5 billion years to travel from the boundary of ...
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Is every object in the universe really expanding? Even small scale objects like humans? [duplicate]

So I was asked a question by someone that "if everything in the universe is expanding, then the lens we use to look at stellar objects is expanding as well, meaning we shouldn't be able to tell the ...
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Is a vacuum the same as 'outer space'?

Is a vacuum the same as 'outer space'? When The Big Bang is described the terminology used is that 'space (itself) is expanding'. If that's true what happens when the volume of a container, '...
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Where does the 379,000 year recombination prediction of the Big Bang theory come from?

The Big Bang theory postulates that recombination happened 379,000 years after the Big Bang. However, I have never seen a single statement as to how this number came about. I don't imagine they ...
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Linearly expanding vacuum solves cosmological constant problem? [duplicate]

The flat FLRW metric written in terms of conformal time $\eta$ is given by $$ds^2=a^2(\eta)(d\eta^2-dx^2-dy^2-dz^2)\tag{1}$$ where the scale factor $a(0)=1$. The interval of proper time $\tau$ ...
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Field equations with cosmological constant

Is it enough to add the cosmological constant in the einstein field equations to guarantee that the obtained solution describes an expanding universe?
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Does an expanding universe assume a space within which the expansion is happening? [duplicate]

When I read about the expanding universe and inflation I often see pictures like this: (Source: https://en.wikipedia.org/wiki/Inflation_(cosmology)) or explanation like this: For getting an idea ...
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$c$ slowing down rather than universe expanding?

Can the Friedmann equations be reformulated with taking the universe as being rigid and instead have the vacuum-speed of light in the void decrease over time? So, not changing any equations, just ...
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Universe expanding - Interstellar space, Intergalactic space or Intersupercluster space? [duplicate]

When we say that the universe is expanding, the space between everything is not. The space between atoms, the space between my house and school, and the space between Earth and Sun remains the same, ...
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Is the acceleration of the expansion of the universe caused by expansion of spacetime itself?

When considering the accelerating expansion of the universe, it is often stated that dark energy is required to cause acceleration of expansion in order to overcome the pull of gravity. However, it ...
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Newton vs. Einstein in universe evolution description

The equation for the universe evolution in the presence of a perfect homogeneous fluid, namely $$\dot{R}^2+k=\frac{8\pi G}{3}\rho R^2$$ can be obtained by solving Einstein equation with some effort. ...
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Speed of at expansion at the start of the big bang [duplicate]

If I understand the basics of the big bang correctly it starts with a very small dense point. At the moment of the big bang, less then microseconds after, the universe is already the size of our solar ...