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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Big Rip: will cosmic event horizon's Unruh radiation balance accelerating expansion?

The way I've seen the expansion of the universe described, it seems to work a bit like an inside-out black hole: an event horizon beyond which there is no causal connection, light from beyond it can't ...
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Expansion of the time dimension [duplicate]

Given that the universe is expanding in the three spatial dimensions, is it possible that the time dimension is also changing over time? If the rate of the passage of time is changing over time, ...
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No Temperature in an Expanding Universe?

The statistical definition of temperature as $\bigg(\dfrac{\partial \ln \Omega}{\partial E}\bigg)^{-1}$ inherently assumes the existence of a well-defined energy. To my understanding, a well-defined ...
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Freeman Dyson: Advanced intelligences thinking in an expanding universe [closed]

I dont know if this is the correct place to ask this, because is highly speculative and maybe even hard scifi. But... I think is less speculative than the many worlds interpretation of QM or many ...
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Why is inflation described by a scalar field?

Textbooks on cosmology describe the phenomenon of cosmic inflation in terms of the existence of a scalar field (or many scalar fields, as the answer and comments pointed out), called inflaton. Such as ...
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What is the limiting factor of the speed of light?

Have been really struggling for some considerable time to understand the universal limitation of speed i.e. it is not possible to travel faster than the speed of light. In particular - what is the ...
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Understanding expansion of the universe

I'm having problems to understand "graphically" what is the expansion of the universe. Does it mean that if now we have two points A and B separated by 1 meter, in a future those points will be ...
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Does a zero-energy universe imply a Big Crunch?

My reasoning is that since gravity is the only source of negative energy in the universe (under the assumption that all the other forces eventually cancel out without any overall effect) is ...
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Applicability of time-dependent metric for expanding universe

What makes us believe that a time-dependent metric is the right way to describe expanding universe instead of just claiming that galaxies fly away from each other? I can see, that it allows them to ...
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Angular Diameter Distance to the last scattering surface

The definition of the angular diameter distance is the ratio of an object's physical transverse size to its angular size. However when I was reading my textbook, Astrophysics in a Nutshell by Dan Maoz ...
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Does the expansion of the universe affect its shape?

This may be a very simple question, yet I cannot find an answer that satisfies me. I have read (and please correct me if I am wrong) that the shape of the universe depends on its density. If it has ...
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Does the Uncertainty Principle imply a linear cosmology?

If the uncertainty in the age of the Universe is $\Delta t$ then the Uncertainty Principle implies that it has an uncertainty in its energy $\Delta E$ given by $$\Delta E \ \Delta t \sim h.\tag{1}$$ ...
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How certain do science believe the universe is expanding?

The continue and increased rate of expansion is backed up by red shift, but that can also be true for a slowing down universe. Can the acceleration of expansion be reducing, still causing red shift ...
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Can we use redshift measurements to determine absolute velocity?

Here's a thought experiment: suppose I'm in a large box in space without windows or sensors, and I fire a laser in 6 different directions and measure the redshift along each direction. Could that data ...
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Energy conservation on expanding universe [duplicate]

Due to the expansion of the universe, the photons emitted by the stars suffer redshift, Its mean that the energy is lowered a little bit. Does this mean that the energy is lost? Does the expansion of ...
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Dynamical Casimir Effect in expanding space?

The Dynamical Casimir Effect is the production of real photons from the vacuum in a system where one has moving mirrors. The frequency of the photons is related to the ratio of the velocity v of the ...
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Expansion of Universe: Spacetime metric and Topological Variations? [duplicate]

There are many posts, questions, comments and excellent answers on this website pertaining to cosmology, expansion, universe, spacetime et cetera. Yet, the following question was not asked or answered,...
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How can cosmological constant serve two opposite purposes?

Einstein used the cosmological constant to prevent expansion which he regarded as his greatest blunder. On the other hand, we are saying that the cosmological constant can cause the space expansion to ...
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Was the expansion of the universe ever decelerating?

The Universe has always been expanding since the big bang. Today the expansion is accelerating. Has there been any point in time when the expansion of the universe was decelerating? How do we know ...
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Will a closed universe with dark energy still collapse into a big crunch or will it expand forever?

In a closed universe without dark energy, it departs rapidly from flatness and become more curved over time. The expansion of the universe eventually stops and starts to collapses into a big crunch. ...
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Space expansion and speed of light

I recently saw a video on gravitational waves that says that expansion of space can only be measured due to changes in speed of light as everything else that could have been used to measure the ...
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359 views

Redshift - How can Z be greater than 1?

I'm having trouble understanding the equation for redshift: z = Δλ/λ ≈ Δf/f ≈ v/c If z = v/c and c = speed of light, how can z>1 (as nothing can exceed the speed of light)?
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Space expansion and conservation of energy [duplicate]

The phrase of 'expansion of universe' , as we are led to believe, is mostly referring to an expansion in space. But is space expanding in the sense that the space between two particles is increasing ...
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Why does the Hubble constant have a subscript zero?

The title says it all. Why is the Hubble constant written $H_0$ and not just $H$? What is the purpose of that subscript $_0$?
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Is a closed universe symmetric?

Say our universe is closed, at some point in the future it will reach a maximum and then begin to contract, will it return to say the point we are now in exactly the reverse manner? For example, as ...
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Can a closed universe become open?

My feeling is that no a closed universe can't become open. Under normal evolution according to the Friedman equation the curvature would be more exagerated as time goes on (i.e. a a closed universe ...
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Could this experiment show how big the universe really is? [duplicate]

Thought experiment: Entangle a pair of particles. Package one half of each pair in a special package, and send it out in a nano-spaceship keep the other half of the pair of entangled particles here ...
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Is it possible to experimentally show that observable universe is much larger than ~93B ly?

Thought experiment: Entangle a pair of particles. Package one half of each pair in a special package, and send it out in a nano-spaceship keep the other half of the pair of entangled particles here ...
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Recession velocity, redshift and different cosmological models?

Equation 1 (page 5) in “Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the universe” by Davis and Lineweaver gives the general relativistic ...
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Is Heat Death the only inevitable end?

I have heard about heat death being the most probable end of the universe. But what are the other possible ends of the universe. I also came across a video stating that due to some effect called as "...
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The universe is speeding up?

I just watched a show on spacetime. In the show they said that the universe's expansion is speeding up instead of slowing down. They said it had to do with dark energy. I was wondering if you could ...
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Differentiating cosmological redshift wrt time confusion

I'm trying to understand Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the universe by Davis and Lineweaver. On page 19 they give an equation (...
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1answer
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Is the mean CBM frequency more red-shifted since discovered in the 1960's?

If the universe is uniformly expanding at rate x, is possible to measure that the CBM has increased its red-shift value over time? If the red-shift is a static value, is it possible that the ...
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How much has the universe expanded?

Expansion speed of universe is 68 km/sec, which is 1/4400 times the speed of light. From the big bang 13.7 billion years ago, the universe would have expanded a few hundred million light years at ...
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Why does the current age of the universe increase when dark energy is included? (open universe)

When numerically solving the Friedmann equations for varying cases of an open universe (i.e. $\Omega_0 < 1$) I get the following evolution plots, where the left plot is for an open universe ...
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Does the location of the cosmic horizon depend on the local speed?

In the current cosmological model the observable universe is limited by a cosmic horizon. We can't see galaxies beyond the horizon. Imagine a spaceship in the vicinity of our horizon. If the ship is ...
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Could dark matters massive gravity create areas of slowly released expanding space left over from the big bang? [closed]

I am not a physicist and apologize if this wastes anyone's time. I follow progress in most theoretical fields of physics and understand the premises but not how they are realised. The question I posed ...
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Does matter follow the expansion of space?

Non-physics person here. I am trying to understand the expansion of space. I've read about the redshift of light and the balloon model (2 points on expanding balloon move away from each other). The ...
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Isn't A Singularity Inevitable Given Infinite Time?

I am not a physicist but this is a question I've been trying to find the answer to for years and no answer I've been given has satisfied me. It's my understanding that gravity affects all matter all ...
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2answers
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Does our Galaxy move at the speed of light? [closed]

I am not a physicist, but I have a few questions that bother me for a while: As we do not know if the Universe is infinite or not I assume we do not know our whereabouts in it; We know,though, there ...
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Does length contraction apply to the far galaxies moving due to expansion?

From Wikipedia: Universe in an expanding sphere. The galaxies farthest away are moving fastest and hence experience length contraction and so become smaller to an observer in the centre. Does ...
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Friedmann equations in a polar axial anisotropic universe

The first Friedmann equations which describes the expansion and content of the universe is as follows: $$ H^2 = (\frac{\dot{a}}{a})^2 = \frac{8\pi G}{3} \rho - \frac{kc^2}{a^2} + \frac{\Lambda c^2}{3}...
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For how long CMB was being emitted?

Or equivalently, how long was the recombination period? Was it a fraction of a second? Millions of years?
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The Space Between Us [duplicate]

We are told the universe is expanding at anything between 68 and 73km per second, per megaparsec (depending on the places you look) - so by association, the space between us and anything else out ...
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1answer
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Inertial mass increase of the farthest receding galaxies

If the farthest galaxies that we can see are receding at near the speed of light, shouldn't this increase their inertial mass as well relative to our galaxy?
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The difference between comoving and proper distances in defining the observable universe

"The radius of the observable universe is estimated to be about 46.5 Gly." If I understand correctly, it means the most distant object that we can see right now is 46.5 Gly away (at present), but it ...
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How likely are Primordial Black Holes to form in the early universe?

In the very early universe, tiny fluctuations created slight over- and under-densities in mass/energy. As far as I understand, if a region becomes sufficiently over-dense, a primordial black hole (PBH)...
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If the expansion of the universe slowed what would we observe?

I was reading Universe is expanding at enormous speed and was wondering built on from the answers given, about how the expansion of the universe is not limited to the speed of light, the rate of ...
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Electromagnetic action in the expanding universe

In the static Minkowskian spacetime the action for the electromagnetic potential in the radiation gauge is given by $$ \mathcal{S}=\frac{1}{2}\int~dt\int~d^{3}x\left(\left(\frac{\partial\textbf{A}}{\...
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Does localized space contraction occur?

There's the general space expansion due to dark energy and it acts in areas that aren't gravitationally bound. There's gravitational distortion of space around massive bodies, which also adds space. ...