Questions tagged [cosmological-constant]

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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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Quintessential models for dark energy

Following Sean Carroll here There are good reasons to consider dynamical dark energy as an alternative to an honest cosmological constant. First, a dynamical energy density can be evolving slowly ...
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1answer
69 views

Is there a cosmological constant $\Lambda$ in 2D?

It is well known that the Einstein tensor is trivially 0 in any two dimensions spacetime: \begin{equation}\tag{1} G_{\mu \nu}^{(2)} \equiv 0. \end{equation} Thus, it is not possible to formulate the ...
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Minkowski space vs De Sitter Space

Can you explain to a layman the differences between the former two?
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3answers
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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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Fine tuning of the cosmological and gravitational constants [closed]

Consider the gravitational constant $G$. According to philosopher Robin Collins, cosmologists have been able to determine a region $W_r\subseteq\mathbb{R}$ for which the universe would have permitted ...
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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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0answers
62 views

How come the universe is considered flat if zero point energy is infinite?

If quantum field theory calculates that the vacuum energy is infinite and Einstein's theory of gravity implies this energy should produce a curvature of space-time then why shouldn't the universe be ...
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1answer
74 views

Maxwell equations in curved spacetime with cosmological constant

I am aware of Maxwell Equations in Curved Spacetime. But how do these equations change if the cosmological constant is not assumed to vanish?
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What's the difference between “dark energy” and “vacuum energy”?

Let's see if I understood well. "Dark energy" is a term that encapsulates everything that may be accelerating the expansion of Universe, and "vacuum energy" is a kind of energy that permeates the ...
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35 views

Justification for “running” cosmological constant

I am looking for papers which seek out to justify that the cosmological "constant" could be time-dependent. I am aware that within the framework of GR the cosmological constants really has to be a ...
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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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1answer
24 views

Hubble Parameter in terms of Density Parameters

So I have the equation (derived from the Friedman equations with $a(t_0)=1$ and the powers of $a(t)$ quoted)$$H^2 = H_0^2\left(\frac{\Omega_{R_0}}{a^4(t)}+\frac{\Omega_{M_0}}{a^3(t)}+\Omega_{\Lambda_0}...
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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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Friedmann equations and the age of the universe

We have Friedmann equations of the form \begin{equation} \left(\frac{a'}{a}\right)^2 = \frac{8\pi G}{3}\rho + \frac{\Lambda}{3}. \end{equation} We also assume that the stress-energy tensor has $T^{00} ...
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2answers
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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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What theories purport to describe “physical-level understanding” of dark energy?

What theories propose to close the gap (as suggested by http://arXiv.org/abs/1210.6008) between 'phenomenological behavior of dark energy' and 'physical-level understanding of dark energy' and to what ...
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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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1answer
74 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
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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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56 views

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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53 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
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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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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
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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
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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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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
255 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
100 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
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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
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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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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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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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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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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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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
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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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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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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 ...