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Questions tagged [cosmological-constant]

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What is the relationship between the Cosmological Constant and the Cosmological Principle?

I believe I've misunderstood a relationship between the cosmological constant and the cosmological principle having read: Einstein introduces his cosmological constant which attributes, in the ...
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0answers
38 views

Planck units and other constants

Why is it that only $G$, $\hbar$ and $c$ are used as the three fundamental constants. What about the ratio of the mass of a proton to an electron, or the quantity of dark energy, and so on?
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Is physics as we know it possible in a hyperbolic space? [closed]

I understand that the overall curvature of the universe can take many different values, and as far as we know the actual total curvature for our universe is near 0. I understand it is possible ...
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1answer
70 views

What are the units of dark energy?

Popular literature seems to equate dark energy with the cosmological constant of the Einstein field equations. We know however that the dimensions of any constituent of the field equations is ${\rm ...
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2answers
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What can I see if I hold a ruler that stretches into a distant receding galaxy? [closed]

If my clones were to form a line along that metric ruler that bridges Milky Way galaxy and a still red shifting galaxy far far away, would any of me notice anything about the scale of the ruler? Now ...
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1answer
83 views

What's the definition of repulsive gravity?

After asking this question, I got confused. I always thought that repulsive gravity was a consequence of dark energy, the Nature of which is unknown. Many theories are flying around about its Nature. ...
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General covariance and a running cosmological constant

A running cosmological constant $\Lambda(t)$ can always be included in the perfect fluid source tensor. By the transformation properties of tensors Einstein's field equation is still independent of ...
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0answers
48 views

How accurate is the De Sitter solution of the Einstein field equations of general relativity?

The De sitter solution models the universe as spatially flat, neglecting ordinary matter, such that it is dominated by the cosmological constant. The cosmological constant notion arguably suggests the ...
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2answers
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Does Higgs Boson influences with Gravity?

Does Higgs Boson influences with Gravity? Is there any way that if the universe is expanding Higgs particle has influence over it?
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1answer
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Vacuum energy length scale detectable by Casimir effect?

According to Sean Carroll's The Cosmological constant (Eqn.20) cosmological observations imply that the magnitude of the vacuum energy density in natural units is given by $$|\rho^{(obs)}_\Lambda|\le (...
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1answer
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Does string/M-theory provide a solution to the dark matter/energy problem?

Would string theory or any other propesed UFT help explain, at least in theory what the other 95 percent of matter and energy in the universe are about? It seems as thaugh there is a reacurring theme ...
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2answers
90 views

Are $2$ and $1/2$ universal constants? [closed]

For example, if the equation for energy were: $$E = mc^{2.713397972993}$$ clearly $2.713397972993$ would be a universal constant. And in the Einstein field equation: $$R_{\mu \nu} - \tfrac{1}{2}R \...
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1answer
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Fine Tuning of the Universe

I'm an A level student looking into the fine tuning of various constants. Physicists explain the extensive effects that would happen if these constants were to be changed/different and hence, how this ...
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7answers
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Strong empirical falsification of quantum mechanics based on vacuum energy density

It is well known that the observed energy density of the vacuum is many orders of magnitude less than the value calculated by quantum field theory. Published values range between 60 and 120 orders of ...
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1answer
67 views

Solving differential equation for the Schwarzschild metric with cosmological constant

How do we solve for Einstein's equation in the vacuum with a cosmological constant, in the static spherically symmetric situation? Attempt: Following Sean Carroll's Spacetime and Geometry (p.195), I ...
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2answers
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Could the cosmological constant be due to vacuum fluctuations in a box, i.e., in a finite universe?

Assumption: If the universe were a finite box whose boundary is the cosmological horizon, then there would be a zero-point energy inside that box. Consequence 1: This zero-point energy would be given ...
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4answers
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What would happen to de Sitter universe if the cosmological constant disappears?

De sitter universe is dominated by a cosmological constant (devoid of matter and radiation), corresponding to dark energy in the far future or the inflaton field in the early universe, leading to an ...
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1answer
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Why do we interpret the accelerated expansion of the universe as the proof for the existence of dark energy?

Why do we interpret the accelerated expansion of the universe as the proof for the existence of dark energy? The accelerated expansion only tells us that the Einstein field equation must contain a ...
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3answers
271 views

Does string theory accommodate effects of dark energy?

String theory is arguably, among best candidates for a Theory of Everything. So, because every TOE is an attempt to bind together the big and very small effects and things, a TOE must be able to ...
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1answer
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Is cosmic inflation slow rather than fast

According to cosmic inflation models, there is an early period where $\Lambda$ dominates and the scale factor grows exponentially. Among other things, this helps to give a reason for large-scale ...
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1answer
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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
207 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
91 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
69 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
60 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
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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
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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
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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
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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
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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
37 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
150 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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0answers
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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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6answers
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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
51 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
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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
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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
71 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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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
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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 ...
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1answer
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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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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
144 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
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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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2answers
398 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
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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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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 ...
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3answers
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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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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 ...
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1answer
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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 $\...