Cosmological Definition and 433 Threads

I am familiar with non-Bayesian methods for calculating best fit values of various parametric models, but I have not had any experience with cosmological models calculations. My understanding is that these models have five parameters: H0, Ωr, Ωm, Ωk, ΩΛ, and the last four satisfy the constraint...

We need to prove that a3(t)= ρo/2Λ [cosh(sqrt(24πGΛ)*t) -1] by changing into a variable of u, where u=2Λa3/ρo From Friedmann's second equation we know that Λ= ρm/ 2 Also ρm= ρo/ a3 [First attempt] I begin from Friedmann's equation where (for here), ρtotal= ρm + Λ and k=0; a'2/a2 = 8πG(ρm +...

ρο/a4 ρο/a3 k Λ Βig Bang Big Crunch Polynomial Expansion Exponential Expansion x 0 x 0 >0 x +1 x +1 x 0 <0 x -1 x -1 >0...

I am confused about the cosmological constant and dark energy. In the most accepted theory, energy is created as the vacuum of space expands. This contravenes the conservation of energy. The law of conservation of energy does not hold in curved spacetime but isn't our universe flat spacetime ...

The universe will expand exponentially when dark energy completely dominates the energy density of the universe. To clarify, does that make the present expansion 'quasi-exponential'?

Hi everyone, I'm unable to understand how to derive Formula (6.3.11) in Weinberg's cosmology book. It's a relation between time-related derivation (d/dt) and RW-scale-factor-related derivation (d/dy, where y = a(t)/aEQ, a(t) is the RW scale factor in the metric and the EQ subscript denotes the...

Homework Statement Consider a point in the intergalactic medium at some cosmic time ## t_{obs}##, the time of arrival of a photon of wavelength ##λ_{obs}## as seen by a hydrogen atom at that location. The source of this photon a comoving distance ##r## away emitted it at wavelength ##λ_{em}##...

One side of the Einstein Equations with a cosmological constant is ##R_{\mu \nu} - (1/2) Rg_{\mu \nu} + \Lambda g_{\mu \nu}##. Question is, why the cosmological constant term appears with a plus sign and without a factor of 1/2 in front of it? I guess it may be because ##\Lambda## is (in...

Let us suppose we have a metric in the form of, $$ds^2=-c^2dt^2+[(a^2(t)+b(r)e^{-lt})(dr^2+r^2d\Omega^2)]$$ Where scale factor is defined as ##(a^2(t)+b(r)e^{-lt})## Is this metric describes homogeneity and isotropy or not ? I think it cannot since there's an ##r## dependence, and there are...

What causes Cosmological redshift? Can it be due to Compton scattering with free electrons in the corona/atmosphere?https://en.wikipedia.org/wiki/Corona

I'm curious to know whether anyone with good maths has anything to say about Dr Philip Gibbs' covariant formula for conserved currents of energy, momentum and angular- momentum derived from a general form of Noether’s theorem? I'm not a pro mathematician, but it looks relatively robust to me...

Allegedly, string theory (in it's simplest form) predicts that cosmological constant must be negative (or zero). Can someone explain where does this result come from? A reference would also be welcome.

Was watching some documentaries and got confused about something. People say that Einstein unintentionally predicted that the universe was expanding, and that he inserted the cosmological constant to represent a force pulling it back in. But other sources seem to imply that the universe was...

Hi, I want to be sure i understand correctly the implications of problem 2 in: http://universeinproblems.com/index.php/Time-dependent_Cosmological_Constant "when G is constant, Lambda is also a constant if and only if the ordinary energy-momentum tensor Tμν is also conserved " As far as i...

Am I correct in understanding that locally, ie, with respect to circumscribable phenomena, conservation of energy is valid in the cosmos but that otherwise it is/may not be? Otherwise said, the source of dark energy does/may not obey this principle? Or is this a question that does not fall...

Has anyone conjectured that the redshift associated with Hubble's constant can be explained by Heisenberg's uncertainty principle? Another words, the further in space away from us a photon is emitted from a galaxy, the longer in time it takes for the photon to reach us. Since the photon has a...

I understand that andromeda is blue shifted because it is moving towards us, but do we see any kind of redshift because of the expansion of the universe. Is andromeda less blue shifted than it should be because of cosmological redshift?

So I recently had a conversation with a mathematician friend of mine who studies Einstein's equations, and he asked me this: Why do physicists call it "dark energy"? It isn't like the dark matter problem, where there is almost certainly some massive "stuff" out there gravitationally influencing...

Homework Statement I am unsure of Q3 but have posted my solutions to other parts Homework EquationsThe Attempt at a Solution 3) ok so it is clear that because the metric components are independent of ##x^i## each ##x^i## has an associated conserved quantity ##d/ds (\dot{x^i})=0##. (1) The...

For instance, in introduction in https://www.sciencedirect.com/science/article/pii/S0550321314001400 we can find that vacuum energy is proportional to ##k_{max}^4## where it is written that "If we believe the general relativity up to the Planck scale ##k_{max}=10^{19}GeV##" And so the...

Hi all, I am trying to understand a few basic concepts about cosmology and the CMB, but I am not getting the information presented in these graphics. I've seen them a million times, and I generally just take it for granted without close examination, but now it's bothering me. (these come from...

NASA decided to stop *WFIRST and concentrate on Mars project (send humans to Mars). What do you think about it? In my opinion, since a lot of ventures have begun space developments such as SpaceX and this proves that rockets make money, NASA should tackle WFIRST. Generally, we cannot earn...

I am trying to find a paper which explains the derivation of the Cosmological Constant. I looked several books and sources and it only says " The cosmological constant ##Λ## appears in the Friedmann equation as an extra term" or etc. Sometimes It directly puts it in the Friedmann Equation which...

In studying the chapters on cosmology in Misner/Thorne/Wheeler (what a textbook!), I see that the cosmological redshift is different than Doppler. If I understand correctly, it is due to the expansion of the (Friedman, FLRW) universe during the photon’s long journey to us, rather than being due...

Does the presence of the cosmological constant modify the rate of expansion of the universe even during the earlier deceleratingly expanding phase of the universe?

Is there more to our universe than what we can observe? If so, does that mean that photons from the CMB are traveling towards us from beyond our cosmological horizon?

Under the holographic principle, the physical description of the 3d world at a particular location in spacetime is encoded on the two dimensional cosmological horizon that encapsulates it. Does that imply that the "measurements" taking place on a 2d shell that is the horizon is creating new...

In his 2005 paper titled 'Lifetime of the universe1' [ https://arxiv.org/abs/hep-th/0510003 ] Canadian physicist Don Page gives an argument that our universe must end on the timescale of 1060 years to avoid having more Boltzmann brains than normal observers. If not the volume of our comoving...

Why is it the case that, in a semiclassical description of the Einstein-Hilbert action, the cosmological constant is small in Planck units? Why does this mean that $$\ell \gg G$$ for ##\Lambda = - 1/\ell^{2}##?

Let me preface this post by saying that I only have a very cursory understanding of general relativity. I happen to know that if we assume the cosmological principle, then the hypersurface ##\Sigma_t## of the spacetime manifold ##M##, for any positive ##t##, is either a 3-sphere, a...

My question is about the interpretation of the large estimated value. In QM we are supposed to think in terms of measurement results and not of ontological properties. So, if QFT predicts a large vacuum energy what is the correct approach? 1. The predicted value is the result you get if you...

The Lagrangian density for cosmological constant is $${\cal L} = \sqrt{g}\Lambda$$ Let us write, schematically, $$g=\eta+h$$ where ##\eta## is the flat Minkowski metric and ##h## is the spin-2 field. Expanding the square root for small ##h## we get something like $${\cal L} = \Lambda + h\Lambda...

Som news outlets are reporting a potential solution to the cosmological constant problem: https://arxiv.org/pdf/1703.00543.pdf opinions on this paper are much appreciated.

Is the Hubble constant decreasing over cosmological timescales?

Hello! i am an undergraduate and I started working with a professor doing numerical simulation of cold dark matter. I understand (more or less) the physics behind and the results are very close to observations, which is a good support for the existence of dark matter. However, I can't see the...

There is Hawking radiation associated with black hole event horizons. And there is Unruh radiation associated with horizons produced by acceleration. I've also heard some suggest that there is radiation associated with the cosmological event horizon due to space itself accelerating in its...

https://phys.org/news/2017-01-cosmologists-closer-quantum-gravity.html Researchers from the University of Portsmouth have revealed quantum imprints left on cosmological structures in the very early Universe and shed light on what we may expect from a full quantum theory of gravity. Dr Vincent...

I began thinking about the problem I describe below from trying to understand the discussions in another thread. https://www.physicsforums.com/threads/concordance-cosmology-paper.909603 This problem is about the expansion behavior of a “simple” universe model that might demonstrate a distinction...

what is thought to be the closest star whose light visible to us now is/has cosmological redshift? perhaps the same answer but in case not: at what LY distance does the prevalent cosmological redshift of distant starlight begin? TY, r

Hello guys, I have a question that came up to my mind while thinking about the evolution of our universe and quantum mechanics. It involves cosmology so maybe that was the right spot to post it, and of course the moderators can do it if they feel it's more adequate for the thread, but my...

As the Universe expands and the photons stretch to longer wavelengths (say from UV photons to the CMBR now) where has the energy gone since longer wavelength photons have less energy? Regards, G.

Cosmological paramaters in the cold dark matter Lamda concordance model. I have been trying to find the most uptodate values for Ho , Ohmega m and Ohmega Lamgad I was wondering if anyone knows where I might find the most reliably, widely accepted values for these values that are currently...

How is entropy defined (if it is) for phenomena taking place on a cosmological scale? Entropy in thermodynamics is defined for equilibrium conditions. Do we assume cosmological phenomena are approximated by equilibrium conditions?

There is a question that has been bugging and puzzling me for a long time and I wondered if somebody could help me to figure out what the answer is or where I am not thinking straight. Let us assume that the standard model of particle physics is more "right" than "wrong" and there is no such a...

is there any limit to the expansion of the universe? (Cosmological Expansion) Can anyone refer to some research papers on this topic?

I have read this paper, but i do not understand the consequences in the broadest sense.arXiv:1612.02449 [pdf, ps, other] Cosmological constant vis-a-vis dynamical vacuum: bold challenging the ΛCDM Joan Sola Comments: 31 pages, 2 tables, 9 figures. arXiv admin note: text overlap with...

in 2 recent papers by Lee Smolin and Erik Verlinde it is claimed that the scale of acceleration as required by various MOND theories is on the same order as the cosmological constant both researchers suggest that instead of cold dark matter, or even modified Newtonian dynamics, MOND-like...

Based on this lecture notes http://www.helsinki.fi/~hkurkisu/CosPer.pdf For a given coordinate system in the background spacetime, there are many possible coordinate systems in the perturbed spacetime, all close to each other, that we could use. As indicated in figure 2, the coordinate system...

What is the main motivation behind the theory of cosmological perturbations? Is it the observational data, observational hints, or perhaps the theory of inflation?

It is common is cosmology to study density fluctuations in the early universe. However, it is also common to assume that the background space is homogeneous and isotropic and use the FRW metric. I do not see how density fluctuations can be possible in a homogeneous and isotropic space. Can you...