Questions tagged [cosmological-constant]

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How do quantum fluctuations make up vacuum energy? [duplicate]

Quantum fluctuations are thought to be the reason why we have energy even if there is nothing in a region of space. My question is if quantum fluctuations give back what they got from nothing using ...
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Variable Cosmological Constant - Friedman Equations

Are the Friedman equations which are a solution of the Einstein Field Equations also applicable to cosmologies where the Cosmological Constant varies with time? Also is one allowed to move in the ...
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337 views

What is the exact relation between the age of the universe and the cosmological constant?

I understand that the relation between the age $t_0$ of the universe and the cosmological constant $\Lambda $ is something like $$c t_0 = \frac{f}{\sqrt{\Lambda}}$$ Can somebody provide the precise ...
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118 views

The problem of General relativity

Did Friedmann by his equations find a problem in the theory of general relativity? Because at first Einstien built his theory on a steady state universe. But, Friedmann according to this theory ...
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Is the cosmological constant the simplest candidate for dark energy?

I'm reading this article on Dark matter and dark energy. There is a statement in the document which is as follows What is the best bet for the nature of dark energy? ...The simplest candidate ...
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Does string theory give any values for the predicted “leakage” of gravity into extra spatial dimensions?

Sorry if this is a duplicate. I have read that string theory has a good explanation for the "hierarchy" problem of the force strength disparity between the fundamental forces because as gravity ...
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91 views

How can the expansion of the universe accelerate?

If according to Newton’s 3rd law (action = reaction) the pressure to expand the universe, to accelerate galaxy clusters away from each other never can exceed nor precede in time their opposition to it ...
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1answer
333 views

Anti-de Sitter space in an embedding space

The Wikipedia page on anti-de Sitter space defines such a space as the hypersurface $$- X_{1}^{2}-X_{2}^{2} + \sum_{i=3}^{n+1}X_{i}^{2} = \frac{(n-1)(n-2)}{2\Lambda}$$ in an $n+1$-dimensional flat ...
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Why is the cosmological constant on the left of the EFE?

My confunion stems from the common knowledge that: in the Einstein Field equations, the terms are ordered so that terms for the matter and energy are on the right, and on the left you find the terms ...
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80 views

Cosmological constant in a semiclassical approximation of quantum gravity

Why is it the case that, in a semiclassical description of quantum gravity, the cosmological constant is small in Planck units? Why does this mean that $$\ell \gg G$$ for $\Lambda = - 1/\ell^{2}$?
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259 views

Why is dark energy proof for an open universe?

Dark energy appears to be increasing at the rate which the universe expands. Why does this show the universe is open?
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482 views

Understanding the energy density of the the false vacuum

This note by Alan Guth says that The false vacuum, however, cannot rapidly lower its energy density, so the energy density remains constant and the total energy increases. Since energy is conserved,...
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What is the source of vacuum energy during the inflationary era?

It is assumed that during the inflationary epoch the universe is dominated by the vacuum energy. What is the source of this vacuum energy and how it that related to the energy of the inflaton field?
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Large $D$ limit of (Anti) de Sitter Space is Minkowskian Space?

As is well known, the solution of the vacuum Einstein equations with a non-zero cosmological constant, $G_{\mu\nu}+\Lambda g_{\mu\nu}=0$, is an asymptotically (anti) de Sitter space based on the sign ...
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A Universal Upper Limit on Mass Within a Radius $R$?

Since the universe has a positive cosmological constant, there is an upper limit on the mass of the black holes as evident from the so-called Schwarzschild-de Sitter metric: $ds^2 = -f(r)dt^2 + \...
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Is the Cosmological Constant locally zero?

On earth and in our solar system we do not notice any effects of a non-zero cosmological constant. The accelerating expansion of the universe was only detected by observing the most distant supernovae....
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317 views

A Very Sharp Cosmological Constant Problem In a 1+1 Dimensional Universe?

Equations of GR with a cosmological constant $\Lambda$ read $R_{{\mu}{\nu}}-\dfrac{1}{2}Rg_{{\mu}{\nu}} + \Lambda g_{{\mu}{\nu}} =8\pi T_{{\mu}{\nu}}$ Therefore, $R-\dfrac{D}{2}R+D\Lambda=8\pi T$ ...
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449 views

Anti de Sitter and FLRW metrics

Perhaps this is a naive question, but I have been reading about conformal invariance and conformally related metrics and I would like to know if someone can clarify me some concepts on this. Anti de ...
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How is the horizon size related to the Hubble parameter or number of e folds of inflation?

I know the following relations $$N=\int_{t_i}^{t_f}Hdt\\ r_{hor}=\int_{t_1}^{t_2}\frac{dt}{a(t)}$$ where N is the number of e folds and $r_{hor}$ is the comoving horizon size. Comoving horizon is ...
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Why the inflaton field?

Why is it necessary to hypothesize a new scalar field, the inflaton field, in order to explain the inflation? In what way, does it differ from the Standard model Higgs field?
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273 views

Basic relativity problem: a limit on the cosmological constant

I am working through a problem in Schutz's A First Course in General Relativity. With respect to Einstein's field equations, $$G^{\alpha\beta}+\Lambda g^{\alpha\beta}=8\pi T^{\alpha\beta},$$ he asks, ...
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Why is the Universe spatially flat even though the various energy densities are nonzero?

This question might be naive for the practitioers of GR and Cosmology. But as an outsider in the field, I could not resist the temptation of asking this. The theory of General Relativity dictates ...
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195 views

Only vacuum is possible in the large $D$ limit of General Relativity?

The Einstein equations with a cosmological constant $\Lambda$ read as: $R_{{\mu}{\nu}}-\dfrac{1}{2}Rg_{{\mu}{\nu}} + \Lambda g_{{\mu}{\nu}} =8\pi T_{{\mu}{\nu}}$ Therefore, $R-\dfrac{D}{2}R+D\...
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265 views

If kappa (Einstein's constant) depends on Lambda (cosmological constant), then why does kappa stay the same when Lambda is not null? [closed]

Regarding Einstein's constant (kappa): Einstein's constant It looks like it was solved based on the assumption the cosmological constant (Lambda) was null: calculations But when Einstein considered ...
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684 views

Schwarzschild-anti-de-Sitter solution and the conformal boundary

The solution of the Einstein field equation with the negative cosmological constant $\Lambda = -3k^2$ or the Schwarzschild-anti-de-Sitter solution is: $$ds_4^2=-\left(1-\frac{2m}{r}+k^2r^2 \right)c^...
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If the distant universe is expanding faster, does that mean that expansion was greater in the past, and is now slowing down?

The predictions of dark energy tell us that expansion is increasing. How can this be true when observations imply the opposite? Why can't Doppler effects be attributed to the state of the Universe at ...
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1answer
312 views

What is the energy of vacuum, plus or minus?

I am learning QFT. If we used scalar field, the energy of the vacuum turns out to be $+\infty$. However, if we use the spinor field, it turns out to be $-\infty$. Both of the infinite energy are ...
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1answer
221 views

How can the permittivity & permeability of the vacuum work as a viscosity term for the speed of light?

I've often read that the speed of light more about the speed of events than light specifically. I wonder if this implies that spacetime has some base viscosity, or mathematically similar mechanic. ...
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340 views

To what accuracy do we know the value of the cosmological constant?

The Wikipedia article just says that the cosmological constant "is measured to be on the order of $10^{−52} \text{ m}^{−2}$," but the only citation goes all the way back to 2004. What is $\Lambda$'s ...
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Did Hubble consider gravitational redshift? [duplicate]

When Hubble first looked at redshifts of light from distant galaxies, and calculated the speed at which they were 'retreating from us' (or, the speed at which the space between them and us is ...
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1answer
105 views

Is the Cosmological constant part of or predicted by or supported by the Dirac Equation?

By Cosmological constant here I mean the constant as would be predicted by the vacuum. The one and the same that if compared to the actual expansion of the universe it is off by a factor of $10^{120}$....
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2answers
579 views

How can Higgs Field contribute to the vacuum energy density if its vev is zero?

I am thinking about all the possible contributions to the vacuum energy. One of them comes from the minimum of the potential of scalar field which is called the the classical contribution. For the ...
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1answer
90 views

Why Astronomers weren't skeptical about the static Universe before Einstein and Hubble?

The Universal law of Gravitation states that all objects are attracted to each other no matter how far apart from each other they are. Isn't that law enough to conclude that a static Universe which ...
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1answer
164 views

Why are only energy differences measurable?

In the framework of special relativity (SR), why is it that only energy differences are observable? Is it simply because, starting our with some action $$S=\int d^{4}x\,\mathcal{L}(x)$$ one can ...
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182 views

The cosmological constant and the precession of Mercury

In the beginning of the last century, the Nobel physics subcommittee – made up of experimentalists detached from the dramatic developments of theoretical physics on the continent – was surprized both ...
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174 views

Do field theories exist for dark energy/matter? [closed]

Has a mathematical theory ever been composed to describe either dark energy or dark matter (or both)? In other words, the standard model describes fermions and bosons; does a model exist to describe ...
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1answer
156 views

How come the flatness problem still a problem while there is an era of cosmic accelertion?

The equation $\Omega-1=\frac{k}{\dot{a}^2}$ together with the assumption that $\ddot{a}<0$ are the equations which demonstrates the flatness problem " because currently $\Omega$ is so close to ...
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243 views

Lemaître's hesitating universes

I'm looking for references on Lemaître's "hesitating" universe models, defined by a long period of stagnation of the cosmological scale factor. These models are using a cosmological constant $\Lambda ...
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1answer
547 views

Constant term in Lagrangian

Why can we always drop any constant term in a Lagrangian density in quantum field theory? This issue is somehow related to the constant term being some kind of cosmological constant. Can you please ...
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Mutiple Zero Point Fields and the Cosmological Constant

Duality of elementary particles like electrons or photons is well known if not fully understood. There is ample research suggesting elementary particles are simply high energy states of universal ...
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368 views

Regardless of the multiverse. How are our constants and laws communicated across our universe, and my bathroom?

Assuming the multiverse theory is true, then each universe has a different set of constants and it provides a reasonable explanation as to why the constants in our universe seem set 'just right' for ...
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How can a cosmological constant be a candidate for dark energy if the universe is flat?

How can a cosmological constant be a candidate for dark energy if the universe is flat? Doesn't a cosmological constant in EFE give rise to positive/negative or no curvature to the universe? I mean, ...
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67 views

Lorentzian Einstein metric where $R_{ij} = g_{ij}$?

Does there exist a Lorentzian metric $g_{ij}$ (with signature $-,+,+,+$) which also satisfy that $g_{ij}=R_{ij}$ (e.g. is Einstein with $k=1$)?
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86 views

Generalising a finding in Kaluza-Klein theory to extra spatial dimensions in super string theory a correct method?

As a disclaimer I am an A-level student. I have been doing a research project in which I am looking at manipulating space-time through the ideas provided by superstring theories. I stumbled upon the ...
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Does anything exist outside of space?

I was doing a little poking around about just what is a massless particle and what that means when I came across this paragraph: If we hop in a spaceship that can go 90% the speed of light, we are ...
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Does empty space have energy?

My physics friend suggested that "the answer to why matter exists in the universe" is because all massive particles are just the fabric of space excited into little packets. To illustrate, imagine a ...
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Could Dark Energy be evidence that space equals energy, in a similar manner to $E=mc^2$?

I was reading another Phys.SE question and answer, and this quote jumped out at me: As for dark energy, the current accepted model, Λ-CDM, treats dark energy like a constant energy density. That ...
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What does the Friedmann Equation imply for the cosmological constant?

The Friedmann equation is given by $$(\frac{\dot{a}}{a})^2 = \frac{8 \pi G \rho}{3} - \frac{kc^2}{a^2} + \frac{\lambda c^2}{3}.$$ Considering that the left hand side is analagous to the kinetic energy ...
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108 views

Before Dark Energy was discovered

I'm really confused on this topic. Pardon my French if my questions seem really stupid. Before Dark Energy was discovered in 1998 did scientists know that the cosmological constant calculated, I ...
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Is cosmological constant really constant?

As the Universe expands, the dark energy in it also increases. I heard that the cosmological constant $\Lambda$ represents dark energy, so that constant must change as time passes, right? Correct me ...