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Questions tagged [cosmological-constant]

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46 views

Can universe still expand without energy and matter?

I heard dark energy is the intrinsic property of space and it cannot be particle because particle's density dilute when volume increase, I do not know how nothingness can have property and Einstein ...
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4answers
519 views

What does the 'cosmological constant' represent?

Newtons theory of gravity involves a gravitational constant $G$, however one does not refer to it directly, we speak instead of gravity or the force of gravity. Now Einsteins introduced a ...
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1answer
109 views

Why is $\Lambda=0$ for a flat matter dominated universe?

I was looking over some of my cosmology notes and was considering the Einstein de-sitter model of a matter dominated universe. I know that in this case that k=0 and also furthermore that $\rho(t)$ is ...
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1answer
70 views

Why does dark energy make universe accelerate not slow down?

If mass=energy, then why doesn't dark energy not make the expansion slow down rather than accelerate seeing as gravity pulls masses together?
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0answers
67 views

Fiber manifold Ricci flat, physical meaning

In a warped-product spacetime, what a physical meaning we have for Ricci-flat Fiber? I'll explain.. it is well known that a Ricci-flat spacetime means that the cosmological constant need not vanish, ...
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1answer
78 views

Non-relativistic limit of the cosmological constant

Usually, when we apply the non-relativistic limit ($c \rightarrow \infty$) to relativistic equations, the cosmological constant $\Lambda \sim \mathrm{L}^{-2}$ is simply offhandedly neglected by ...
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0answers
39 views

Trying to make some sense of the cosmological constant

The question is simple: Does the naive reasoning below have some physical sense, or is it pure gibberish junk? (I use $c = 1$ and metric signature $\eta = (1, -1, -1, -1)$) I suppose that the ...
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0answers
46 views

What kind of average could give a Lorentz invariant energy-momentum tensor?

The electromagnetic (EM) radiation energy-momentum tensor is of the following shape, in the case of incoherent superposition of EM plane waves (I'm using $c = 1$ to simplify things, and metric ...
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0answers
79 views

Natural classification of fundamental constants of Nature

I was wondering about the fundamental constants of Nature since several years, and still pondering on them. Of course, I have read a lot of papers on them, but never found any satisfying ...
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2answers
95 views

Can we set cosmological constant equal to one?

People often say that the cosmological constant is too small. $\Lambda=10^{-120}$ in Planck units. Can we set $\Lambda=\hbar=c=1$ ? If so what would this give for $G$, the gravitational constant in ...
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1answer
39 views

How would we relate the cosmological constant to a dark energy distribution?

*Please excuse my lack of understanding of dark energy, the GR courses that I have taken so far haven't covered it How would we relate the cosmological constant to a dark energy density? Say we move ...
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3answers
210 views

Absorbing the Cosmological “Constant” in the standard Energy-Stress Tensor

Recently I found some publications on Cosmologies with variable cosmological constant. The Bianchi Identity then implies that the divergence of the modified Energy Stress: Tensor $$\hat{T}_{ab}=T_{ab}...
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1answer
170 views

Energy/mass of Quantum Vacuum

So far as we have measured the volume of the universe, has anyone calculated the virtual energy/mass of the Quantum Vacuum? If so, what? Or if not, a local volume such as 1 sq. meter? This relates to ...
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115 views

Meaning of the simplest potential of quintessence models. Fields in denominator?

I am reading Sec. 1.12 of the Cosmology book by Weinberg. In this section he explains the very simple model of quintessence which attempts to provide a dynamical explanation of the smallness of the ...
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1answer
162 views

Does $\Lambda$CDM include inflation?

$\Lambda$CDM obviously includes dark energy (the $\Lambda$) as well as cold dark matter (CDM). Does it also include cosmic inflation? I've not seen a clear answer to this. Wikipedia's article on ...
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6answers
12k views

Why isn't an infinite, flat, nonexpanding universe filled with a uniform matter distribution a solution to Einstein's equation?

In Newtonian gravity, an infinite volume filled with a uniform distribution of mass would be in perfect equilibrium. At every point, the gravitational forces contributed by masses in one direction ...
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1answer
56 views

Does almost constant energy density only require that $ \omega \approx-1 $?

From continuity equation \begin{equation} \dot{\rho}=-3H(p+\rho) \end{equation} $ p+\rho\approx0 $ will give $ \dot{\rho} \approx 0 $ which means almost constant energy density From equation of ...
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1answer
108 views

Is cosmological constant some kind of field that permeates all of space?

One simplest candidate for dark energy is a cosmological constant. Is cosmological constant some kind of field that permeates all of space, or is it just some kind of energy stored in space?
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1answer
66 views

Would the formulation of a theory of quantum gravity in the presence of a nonzero cosmological constant depend on the origin of the latter?

The cosmological constant is the coefficient of a term in the Einstein tensor for which there are no a priori reasons to assume it to be zero, so that it could be regarded as a fundamental constant of ...
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1answer
93 views

Precision in cosmological constant

The observed cosmological constant, what is its precision? How precise do we know is it not a fluke or misinterpretation of cosmological data?
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0answers
45 views

How can energy density (of the same sign) have opposite effects on the curvature of the universe?

Actually I have been inspired by the post Why does dark energy produce positive space-time curvature? to ask the following question. In difference to the just cited post I will take the pressure out ...
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1answer
182 views

Shouldn't the infinite sea of electrons contribute to gravity?

According to my understanding of the dirac equation, there's an infinite sea of electrons occupying all negative energy states which prevents an electron from dropping to lower and lower energy states ...
3
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1answer
76 views

Why should LTB dust be comoving?

In many research papers about inhomogeneous cosmology, one often considers spherically symmetric (LTB) spacetimes where in the co-ordinate frame $(t,r,\theta,\varphi)$ wherein the metric assumes the ...
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0answers
112 views

If the Schwarzschild radius of the universe is bigger than the universe, how come the universe didn't collapse into a black hole? [duplicate]

According to this answer, the Schwarzschild radius of the universe is about 3 times the radius of the universe. According to the Newtonian theory of gravity, inside any sphere of a given density, the ...
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1answer
163 views

Why do we think that the energy density of false vacuum/scalar field doesn’t dilute with the expansion?

I have a question about inflation theory. Why do we think that the energy density of false vacuum/scalar field doesn’t dilute with the expansion? Could it be that the energy density does dilute with ...
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0answers
89 views

Does it ever make sense to write the Cosmological Constant as: $\Lambda_{\mu\nu}$? [duplicate]

In Einstein's Field Equations, does it ever make sense to write the Cosmological Constant as: $\Lambda_{\mu\nu}$? It was written this way in a BBC Horizon Programme on Sunday, But, $\Lambda$ is a ...
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3answers
623 views

Is zero-point energy real?

In the context of canonical quantization, the ground state/zero-point energy of a harmonic oscillator is $$ E_0 = \frac{1}{2}\hbar \omega. $$ The vacuum is alleged to be permeated with this zero-point ...
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1answer
263 views

Why are there a bunch of cosmological coincidences?

There are several cosmological parameters that are more or less of the same order $10^{-60}$ (in Planck unit), namely, inverse of the age of the universe $$ \frac{1}{t_0}, $$ Hubble constant $$ H_0, $...
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0answers
94 views

What conceivable connection is there between the cosmological constant and radius of the Universe?

You probably noticed that the radius of the (observable) Universe in Planck lengths $R$ (about $10^{60}$) and the cosmological constant $\Lambda$ (about $10^{-120}$) in natural units obey ...
3
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3answers
236 views

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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151 views

The theoretical calculation underlying the cosmological constant problem

The electroweak vacuum energy density is $10^{120}$ times larger than the currently observed value of the dark energy. I have two questions in regard to this statement. $\bullet$ Firstly, what are ...
2
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1answer
93 views

Flatness puzzle in a $\Lambda$-dominated Universe

Friedmann equation together with the acceleration equation yield $\dot{\Omega} = (1+3w) H \Omega (\Omega-1)$. The Flatness puzzle lies in the fact that $1+3w>0$, but how is this possible in a $\...
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1answer
218 views

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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1answer
148 views

How did cosmologists calculate the small density of dark energy in our universe?

I've seen the number a few times (138 x10 to the power of -123) but i haven´t found how scientists calculated it. Max Tegmark mentions it in his research paper about multiverse types as representing ...
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2answers
1k views

Why is our physical Universe a de Sitter space?

Wikipedia says, When $n=4$ (3 space dimensions plus time), it is (the de Sitter space) a cosmological model for the physical universe; see de Sitter universe. It appears to me that the statement ...
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0answers
71 views

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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2answers
142 views

Is space in general relativity defined even when devoid of any energy?

General relativity is said to be background independent. Can we nevertheless think of a completely empty space (devoid of any energy) as if it is furnished with an elastic three-dimensional fabric, a ...
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2answers
569 views

Can String Theory really fail to contain a de Sitter vacua?

I was reading a post earlier from Peter Woit's Not Even Wrong blog and came across the following reference to the paper "What if string theory has no de Sitter vacua?" by Ulf H. Danielsson, Thomas Van ...
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0answers
112 views

What if Dark Matter is Majorana Fermion?

Few reasons to believe Dark Matter is Majorana fermion: https://arxiv.org/abs/1711.03877: second contra argument by Jordan and Pauli. "A non vanishing zero-point energy of elementary fields should ...
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2answers
396 views

How to calculate the vacuum expectation value of energy momentum tensor of a scalar field?

I have to prove that the vacuum expectation value of energy-momentum tensor of a scalar field (in flat spacetime) can be written as, $$\langle 0|T_{\mu\nu}|0\rangle=-\langle \rho\rangle\eta_{\mu\nu}.$$...
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3answers
106 views

Is the vacuum energy density proportional to the scale of the universe? [closed]

That is, does the vacuum energy density change with the scale $a$ of the universe according to $$ \rho_Λ \sim a $$ ?
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1answer
47 views

Why are distances in cosmology larger with $\Lambda$?

I'm starting to study cosmology on these beautiful notes: http://www.damtp.cam.ac.uk/user/db275/Cosmology/Lectures.pdf . At page 16, when talking about distances in cosmology, the author says that ...
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1answer
53 views

Cosmological constant phase transition?

I recently watched at a talk by Cumrum Vafa in which he stated that the cosmological constant allows us to define a time-scale $T_\Lambda=1/\sqrt{E_\Lambda}$. The time scale of this time is about 10¹¹ ...
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1answer
39 views

Can a De Sitter universe have a detectable rotation?

In De Sitter relativity, the cosmological constant becomes a geometric term with the dimensions of radius, and the universe becomes a pseudo-sphere. Is it meaningful to speak of intrinsic rotation of ...
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2answers
468 views

Stress-Energy Tensor and the Vacuum

The Einstein field equations, $$R_{\mu \nu} + \left ( \Lambda - \frac{1}{2}R\right )g_{\mu \nu} = \frac{8\pi G}{c^4} T_{\mu \nu}$$ relates the curvature of spacetime and density and flux of energy ...
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0answers
150 views

Inflation and dark energy have constant energy density - energy is created out of nothing without any source or mechanism?

I can understand that a photon in an expanding universe can be redshifted and lose its energy by its wavelength being stretched from space expansion. But in the case of inflation and dark energy, it ...
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0answers
208 views

Cosmological constant problem and dark matter theory by Henke

Christian Henke has recently proposed one approach for the solution of the cosmological constant problem and of the dark matter dynamics: Quantum vacuum energy in general relativity Abstract: The ...
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1answer
76 views

How is expansion of universe seen locally? [duplicate]

Traditionally when deriving Friedmann–Lemaitre–Robertson–Walker metric, homogeneity and isotropy assumption is made. This assumption, however, holds only for large scales. To take into account some ...
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3answers
1k views

Why isn't Anti de Sitter space taken seriously as a model of reality?

I asked a few questions here earlier regarding a physics model which could possibly point to Eternal Return, and was pointed towards Anti De Sitter space. Arguments in theories for Eternal Return ...
2
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3answers
778 views

What if cosmological constant was zero?

Physicists always ask why the cosmological constant is not exactly zero! I would ask here, what if cosmological constant was zero? The universe wouldn't expand and matter would exert gravitational ...