We’re rewarding the question askers & reputations are being recalculated! Read more.

Questions tagged [cosmological-constant]

The tag has no usage guidance.

Filter by
Sorted by
Tagged with
48
votes
3answers
3k views

Are modified theories of gravity credible?

I'm a statistician with a little training in physics and would just like to know the general consensus on a few things. I'm reading a book by John Moffat which basically tries to state how GR makes ...
15
votes
2answers
7k views

Lorentz invariance of the Minkowski metric

As far as I understand, one requires that in order for the scalar product between two vectors to be invariant under Lorentz transformations $x^{\mu}\rightarrow x^{\mu^{'}}=\Lambda^{\mu^{'}}_{\,\,\...
27
votes
6answers
7k views

Do the laws of physics evolve?

Hubble's constant $a(t)$ appears to be changing over time. The fine stucture constant $\alpha$, like many others in QFT, is a running constant that varies, proportional to energy being used to measure ...
26
votes
2answers
3k views

Why does dark energy produce positive space-time curvature?

My understanding is that dark energy, or equivalently a positive cosmological constant, is accelerating the expansion of the universe and I have read that this gives empty space-time positive ...
68
votes
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 ...
3
votes
3answers
617 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 ...
3
votes
1answer
548 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 ...
4
votes
4answers
1k views

What are the differences between dark energy and a cosmological constant?

My possibly mistaken understanding is that dark energy changes with time, whereas a cosmological constant is, well, constant. What about gravitational clumping? Detecting relative motion?
13
votes
1answer
2k views

On Flatness problem, Inflation etc

I have a couple of naive questions from the topic of the title. We know \begin{eqnarray} \Omega-1=\frac{k}{a^2H^2}-\frac{\Lambda}{3H^2} \end{eqnarray} Now I read that from the standard big bang (SBB) ...
2
votes
2answers
2k views

Does spacetime have a “mass” value? or What is “Spacetime” made out of?

When measuring the MASS within the Universe, does "space" or "spacetime" have a value? I only ask, because when speaking of expansion, space is expanding. Could it be possible, to reverse the process,...
0
votes
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 ...
8
votes
3answers
2k views

Vacuum and repulsive gravity

How can one show from General Relativity that gravity is attractive force, and under which conditions it becomes repulsive, also why positive energy vacuum drives repulsive gravity?
6
votes
0answers
2k views

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 + \...
1
vote
2answers
335 views

Why is the cosmological constant a scalar?

Maybe my understanding is just off, but the cosmological constant is just a scalar, right? What are it's units? Why a scalar? - was a tensor 'cosmological constant' ever considered or is it just not ...
6
votes
1answer
543 views

extracting energy from cosmological expansion

This question is a more concrete reincarnation of an old question about energy conservation in GR. Are there mechanisms to extract energy from the cosmic rate of expansion? putting some extremely ...
3
votes
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 ...
1
vote
1answer
323 views

Cosmological Constant on the LHS of Einstein's Field Equation

The cosmological constant seems to be normally described as an energy (repulsive force, Dark Energy) of Space-Time. I was just wondering, if we were to interpret the cosmological constant as being ...
1
vote
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 $10^{120}$....
0
votes
2answers
148 views

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 ...
8
votes
3answers
1k views

G4v Gravity Theory: Why does this get rid of Dark Energy?

Earlier this year, Carver Mead of CalTech published a paper which seems to be garnering a lot of attention: http://arxiv.org/abs/1503.04866 http://www.npl.washington.edu/AV/altvw180.html http://...
14
votes
4answers
1k views

Why didn't Newton have a cosmological constant

Einstein initially added the Cosmological Constant because (if I get this right) it seemed to him that the universe should be static. I agree that back then this would have been an obvious assumption. ...
4
votes
1answer
292 views

Vacuum energy extraction: What happens to the cosmological constant?

If the vacuum energy is extracted, then what will happen to the cosmological constant? Will it remain the same? .
3
votes
1answer
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,...
3
votes
3answers
407 views

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 ...
2
votes
1answer
2k views

Are dark energy and zero-point energy the same thing?

According to Quantum Mechanics is it possible that the famous "dark energy" and "zero-point energy" are the same thing that drives the accelerated expansion of the universe or maybe related to each ...
3
votes
0answers
175 views

Cosmological constant (positive, dS) as SUSY/supertring issue

The "almost" de Sitter expansion of current cosmological model implies a puzzle for supersymmetry (SUSY) and also for string theory. A positive value of the cosmological constant implies a non-null ...
3
votes
1answer
644 views

What is the geometry of our universe?

I have some questions about the notion of space in current cosmological theories. I have been trying to decipher a few papers written on this topic such as this one, but I keep running into some ...
2
votes
1answer
180 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 ...
1
vote
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 ...
0
votes
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
votes
2answers
470 views

Relation between $\Lambda$ and $\Omega_\Lambda$ in $\Lambda\mathrm{CDM}$

In minimal $\Lambda\mathrm{CDM}$, there is a parameter labeled $\Omega_\Lambda$, and current fits place it at around $\left( \Omega_\Lambda \sim 0.73\right)$. Meanwhile, $\Lambda$ enters the Einstein ...
3
votes
2answers
338 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 ...
2
votes
0answers
206 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 ...
1
vote
1answer
116 views

Does the cosmological constant solve the flatness of the Universe with 2 independent methods?

First. The universe is flat and not negativity curved because whatever dark energy is made from represented by the cosmological constant adds to the side of the density in the first Friedman equation: ...
1
vote
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. ...
1
vote
3answers
687 views

Expansion of the Universe: Conversion of gravitational potential energy to kinetic energy?

Suppose there is an object floating in space which over time begins to fall toward the source of a gravitational field. As it falls, its motion happens to be such that it gets locked in orbit around ...
0
votes
1answer
48 views

What is the Komar mass of the de-Sitter spacetime?

The Komar mass of some spacetime is defined as an integral (volume or surface, depending on its formulation): https://en.wikipedia.org/wiki/Komar_mass The de-Sitter metric in static coordinates is ($\...
0
votes
1answer
125 views

Is there any paper analyzing the validity of Relativity in expanding space? [duplicate]

Big Bang models result from applying Relativity equations to expanding space; however, as Einstein noted, Relativity was not established in expanding space. A basic question arises: is Relativity ...