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# Questions tagged [cosmological-constant]

The cosmological constant is the energy density of space, or vacuum energy, that arises in Albert Einstein's field equations of general relativity. It is closely associated to the concept of dark energy, but, according to quantum field theory, it should also account for all the zero-point energies of quantum fields in space.

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### Does the cosmological constant entail a mass for the graviton?

If I consider the Einstein equations into the form $$R_{\mu\nu}=\kappa \left(T_{\mu\nu}-\frac{1}{2}g_{\mu\nu}T\right)+\Lambda g_{\mu\nu}$$ and then linearize them, we should get by moving to ...
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### How to understand critical density?

In Cosmology, critical density is given by setting $\Lambda = 0$ and $k = 0$, in other words, a universe without dark energy and zero curvature. According to my understanding and Wikipedia, this ...
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### If space has a positive curvature, is the expansion of the universe caused by time, not "dark energy"? [closed]

Ok, I will assume that space has a positive curvature, where space is the "surface" of this sphere, and time is the radius from the center, so the universe is a 4D hypersphere. Under these ...
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### Do we really know the universal gravitational constant?

We've all heard $$F_g=\frac{gm_1m_2}{r^2}.$$ However, since I took physics, we've discovered "dark energy," which if I have any concept of the current thinking is caused by space being ...
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### Different types of Dark energy and conservation of energy

According to this Sean Carroll article, and other threads in here, depending on your definition of energy, dark energy does not violate conservation of energy. My question is if this is true ...
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### Why do people say that the $\Lambda$-CDM model has six independent parameters?

The Wikipedia article on the $\Lambda$-CDM model says that the model has six "independent parameters". It also says that the model has several "fixed" parameters and several "...
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### On vacuum energy in a de Sitter universe?

I have a couple of questions about de Sitter cosmological horizons. Initially I made a single post containing the two questions, but after some suggestions, I asked them in two separate posts. This ...
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### The SI-unit of the cosmological constant (vacuum energy) is $\frac{1}{m^2}$. What does that have to do with Energy?

I just don't get how Energy is measured in $\frac{1}{m^2}$. Wasn't it measured in Joules? (source is https://en.wikipedia.org/wiki/Cosmological_constant#Equation)
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### Model of Quintessential Inflation

I have a question about how the potential for an inflaton field is selected. It is clear that there are limitations associated with number of e-folds, scalar-to-tensor relation and scalar spectral ...
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### Cosmological implications of String theory compactification?

Is the process of compactification of hidden dimensions in string theory equivalent to an increasing dilaton field? Would one expect the compactification process to continue indefinitely? Could the ...
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### Sign and physical meaning of the cosmological constant

I've heard that a cosmological constant can be used to model dark energy (e.g. $\Lambda$-CDM model), and that the constant $\Lambda$ should be positive. But my (quite small) understanding of dark ...
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### Solve Friedmann equation for non-zero curvature and non-zero cosmological constant

I tried to find an elegant way to solve (without approximating for low densities) $$\dot{R}^2=\frac{8 \pi G}{3 c^2} \rho R^2-k c^2+\frac{c^2 \Lambda}{3} R^2$$ for $k=\pm 1$ and $\Lambda \neq 0$ (one ...
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### Why are dark matter and dark energy favoured over changes to our physical models? [closed]

I am instinctively skeptical of the existence of "dark matter" and "dark energy". Together, they strike me as being analogous to luminiferous aether -- something that was invented ...
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### Negative $\Lambda$ FLRW spacetimes as infinite black holes?

Consider the Friedmann equation: $$H^2+\frac{k}{a(t)^2} = \frac{\Lambda}{3}+\frac{8 \pi}{3}\rho$$ and set the parameters for dust in either flat euclidean or open hyperbolic spatial slices with a ...
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### Measurement of the Cosmological Constant

Is there some way to measure Lambda, the cosmological constant, independent of $H_o$, the Hubble constant and omega_lambda, the Dark Energy density? A standard equation for calculating Lambda, ...
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### Why is there an infinite supply of energy in slow-roll inflation?

The physical model of inflation includes a metastable false vacuum, or a slow-roll field on a flat potential. In either case, I just realized how this is completely insane. With the exponential growth ...
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### Will cosmic microwave background become invisible in the future?

If my understanding of CMB and Hubble's Law is correct, then CMB photons emitted from more than ~14.4 Glyr during Recombination Epoch would not reach us. The reason is this would correspond to Hubble'...
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### What is the change in vision and measurement between curved space and flat space, especially for measuring cosmic background radiation?

In a curved space, the light bends as it travels and acts like it is going through a lens. In a (positively) curved universe, a small object appears larger. If we know the actual size of an object, ...
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### Big Bang as stretching space time?

I am still new to researching the big bang so please be patient. I am having trouble envisioning the expansion. As I understand under current theory it is not to be thought of as a singularity ...
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### Does energy exist on its own? [closed]

So to my understanding as a layman is that energy transfers from one material to another (I guess that's why there's potential and kinetic energy), for example photons to solar panels. Now my question ...
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### Cosmological constant, dynamical friction and structure formation?

I would like to ask a question about an interesting article that I found (https://repositorio.unesp.br/server/api/core/bitstreams/b8a5a5b8-4b3b-4198-9f5d-bf69431db1ae/content) In the context of ...
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### Mass density and the cosmological constant?

So in flrw metric it's quite reasonable to take eigenvalue of the time-like component of the stress energy tensor and identify it with mass density. Now, if someone argues the cosmological constant ...
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### Question about cosmological constant and radius of curvature of the universe

Dimensional analysis suggests that $\Lambda R^2 \sim O(1)$, where $\Lambda$ is the cosmological constant and $R$ is the radius of the universe. $\Lambda$ is measured to be around $10^{-52}$ m$^{-2}$, ...
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### Right hand side of Einstein field equation

Why can't the RHS of the Einstein field equations take a form like $T_{\mu \nu}$ plus some coefficient multiplied with $g_{\mu \nu} T$? It should also be covariantly conserved, I suppose? For example, ...
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