Cosmological Definition and 421 Threads

So the Universe expanded very rapidly in its very first moments (inflation). The Universe then slowed down and is speeding up again, and Dark energy is supposed to be responsible for this accelerated expansion. The cosmological constant might as well be dark energy, but why is it still being...

Hi everybody! Can you explain me how I can obtain wave equation given a metric? For example, if I have this metric $$g_{μν}=diag(−e^{2a(t)},e^{2b(t)},e^{2b(t)},e^{2b(t)})$$, how can derive the relation $$\frac{1}{\sqrt{g}}\partial _t(g^{00}\sqrt{g}\partial _t...

I am studying cosmolgy and when I thougt about cosmological princples and copernic principle I thougth something.We are observing things and we see the past of objects.From the cosmolgycal principles (isotropy and homogeneity) all observers must see the universe same ,so let's suppose we observe...

Simplistically, the GR equation is G = k T + l g G represents the curvature of the fabric of space-time T is the stress energy tensor, representing the fluxes and densities of matter and energy g is the metric tensor So... In another thread, someone said that the cosmological constant...

So Einstein Equation: ##G_{uv}= 8 \pi G T_{uv} ##, Justifying the cosmological constant can be included is done by noting that ## \bigtriangledown^{a}g_{ab} =0 ## and so including it on the LHS, conservation of energy-momentum tensor still holds. I'm not sure why ## \bigtriangledown^{a}g_{ab}...

...through inertial mass, namely to explain away the sameness of inertial and gravitational mass? If you assume that only inertial mass is a "real" effect, then gravity would simply become a fictious force arising from inertial mass holding matter back from expanding alongside spacetime during...

Maybe this is a complicated question but something I once read on another forum long ago has been rolling through my head lately. In discussing the observational size of distant objects (GLy) a comment was made to the effect that as distance increases an object's size seems to reach a lower...

I’ve read that most comoglocial fluids can be modeled as perfect fluids. And that most perfect fluids obey ##p=w/rho##. I’m wondering (had a look around and can’t seem to find) two things: i) whenever ##p=w/rho## is obeyed does this always give the 3 descriptions of the universe – 2 open...

The ΛEPRL spin foam model presented 25 November at ILQGS by Haggard and Riello achieves an interesting quantization of the cosmological constant. Basically this is done on slide #10 around minute 15 of the audio. http://relativity.phys.lsu.edu/ilqgs/haggardriello112514.pdf...

I am confused about the physical meaning of the redshift. Let say the non-relativistic one z=v/c. When I read Barbara Ryden, Intro to cosmology, she demonstrate the z is not related to the space expansion between the source and the receiver, but 'it does tell us what the scale factor was at the...

I have a problem with some question I had to answer for the Surface Brightness \Sigma \propto \frac{Flux}{Angular~area}= \frac{F}{\Omega}. I was able to show that \Sigma \propto (1+z)^{-4} Then the question asks whether knowing its value for known candles or yardsticks, is a good way to...

There are numerous publications about pairwise velocity dispersion ( PVD ) of galaxies in real redshift surveys. It is customary to use an exponential form for the distribution of pairwise velocities and then model the redshift space distortions in the 2PCF to retrieve the PVD. Now if i have a...

A few small questions: Observationally, why do we conclude that the metric of space is increasing, rather than that light increases in wavelength as it travels cosmological distances? Or are these two conclusions isomorphic? Since wavelength is negatively correlated with energy, where does...

Good evening. I am a current astrophysics undergraduate who is currently having a $10 bet with my classical mechanics professor (chemistry/mathematics background) over what the official curvature of the universe is. While my academic level of understanding is still not quite high enough to...

Homework Statement If light traveled a distance L = H_{eq}^{-1} at M-R equality, how large does this distance expand to at present? (in Mpc) Homework Equations z_{eq} = 3500 \Omega_m = 0.32 at present \rho_c = 3.64 \times 10^{-47} GeV^4 present critical density The Attempt at a...

The title says a lot about my doubt. I don't know a lot about inflation maths but the idea is that in Inflation, I am seeing that the Cosmological Constant (ie Vacuum Energy) is not Constant in two ways: 1 With time; the Vacuum Energy was larger in the beginning and that triggered Inflation...

Homework Statement Give q(t) the deceleration parameter, as a function of: \Omega_{\Lambda}, the cosmological constant density, and \bar{a}(t) = \frac{a(t)}{a(t_{0})}= 1+ H_{0} (t-t_{0}) - \frac{1}{2} q_{0} H_{0}^{2} (t-t_{0})^{2} where a's the scale factors have already defined τ =...

Hi Everyone! I need a bit of help with the following question please. How does the energy density of the cosmological constant Uλ=ρλc^2 have the required SI units if λ=4∏Gρ/c^2 I know that the energy density of the cosmological constant (Uλ)is essentially dark energy and the SI units...

Suppose the energy density of the cosmological constant is equal to the present critical density ε\Lambda = εc,0 = 5200 MeV m-3. What is the total energy of the cosmological constant within a sphere 1 AU in radius? My answer: ε\Lambda = ET / V ET = ε\Lambda * V = (8.33 * 10-10...

Hello, I would like to understand in detail what are the Light-Cone cosmological simulations and the difference between N-body cosmological simulations. If anyone is familiar with the topic i am open to an explanation or perhaps he could recommend me a review or an article about the...

Are dark energy and the cosmological constant the same thing?

The PF home page has some news items in the righthand margin and one is about this: http://arxiv.org/abs/1404.1801 The popular news account is this BBC piece: http://www.bbc.com/news/science-environment-26329320 "They find that some three billion years after the Big Bang, the cosmos was...

The March 2014 issue of Physics Today has an article by Lee Smolin in which he argues that natural laws must change over time. As examples of such theories, he gives Penrose's CCC and his own cosmological natural selection (CNS): http://arxiv.org/abs/hep-th/0612185 My understanding was that...

So I have been thinking. Light gets redshifted because of the cosmological expansion of the Universe. This would mean that other, material particles, should get "cosmologically redshifted" as well right? So, for example, if an electron were flying towards us from some distant galaxy (and we...

Hi, I have some confusion about the cosmological constant (CC). From what I have read it is to do with the energy density throughout the entire universe which is thought to drive the expansion of the universe. If the CC were any smaller then matter would not have been able to form and since the...

Dear every specialists, I heard that BICEP2's observation r=A_t/A_s=0.2 n_t=0 indicate two things: 1) n_t=0 excludes ekpyrotic cosmological model (ECM) definitely because ECM predicts n_t=2 2) r=0.2 excludes almost all current string inflation model (SIM) such as KKLMMT because this...

I wanted to know whether there exists any sort of theory which treats event horizons and cosmological horizons as equivalent. We can never receive any information from inside of a black hole because this would require an object moving faster than the speed of light to escape from the...

Homework Statement Problem in question is problem 5.6 in Dodelson's Modern Cosmology (https://www.amazon.com/dp/0122191412/?tag=pfamazon01-20) Take the Newtonian limit of Einstein's equations. Combine the time-time equation (5.27) with the time-space equations of exercise 5 to obtain the...

In http://arxiv.org/abs/0808.1081 / Am.J.Phys.77:688-694,2009 Bunn and Hogg explain how cosmological redshift can be interpreted as accumulation of infinitesimal Doppler shifts. This suggested to invert z = z(v) and interpret v(z) as relative velocity of two objects with redshift z. It seems...

I have a question about GR in a cosmological setting. If dark energy is assumed to be a true GR cosmological constant, does this require some kind of energy input. I am curious to know if this is just a 'curvature' of space somehow, or does it represent a continual addition of energy on some...

The bare cosmological constant in field theory is needed to cancel the infinite vacuum zero-point energy. Then you get a renormalized cosmological constant. There are three quantites at play, Ω=E+Ω0, where E is the infinite vacuum zero-point energy, and Ω is the renormalized cosmological...

I'm confused about the relationship between two seemingly different concepts of flatness of the universe. 1. Spatial flatness. This is the lack of any curvature on a large scale. Simple enough... 2. Energy density flatness. If the energy density is higher than a critical value, then the...

microwave radiation and the shape of the universe? The hotter parts could indicate parts of the universe that are closer to Earth while the colder parts could indicate the opposite, which would mean the universe is not a perfect geometric shape, which would show no variation in the cosmic...

The cosmological constant (Ʌ ) is equated to dark energy and has units of energy/volume. Why have I read that Ʌ equals the Hubble constant squared (H2) which has units of 1/sec2? There must be an obvious explanation.

I have been trying to pin down a precise definition of large-scale homogeneity, in the context of saying, per the Cosmological Principle, that all constant-time hypersurfaces (CTHs) of a foliation are large-scale homogeneous. Here is my attempt: Let M represent any coordinate-independent...

The Friedmann solution(s) to the GR cosmological equations (without Lambda) assume pressure=0. What happens if we let p>0 ? It seems to me that there is no continuous solution to the cosmological equations in that case. Yet, if we include electro-magnetic radiation (such as MBR) in the energy...

When we look out into deep space we see a red shift in the light from distant sources because they are more or less all receding from us. How is the red shift distorted by passing through our local galactic gravitational well, before hitting us down here on the ground?(more red/more blue/no...

Let's say that an event releases a 1 second burst of EM radiation that propagates outward from the source in a spherical wavefront. If the EM wave travels long enough so that expansion doubles its wavelength, is that 1 second burst received as a 2 second burst? Has the total energy of the EM...

The cosmological redshift can be understood in terms of time dilation. In an expanding Universe light travels on a null-geodesic (ds=0) so that: dr = \frac{c\ dt}{a(t)}, where dr is an element of co-moving distance along its path, dt is an element of time and a(t) is the Universal scaling...

Can anyone point me to some references? Greetings All. I am searching for a couple of numbers, but my books are packed away and stored somewhere due to moving. I was hoping someone could provide the best accepted observational values and the reference sources for those numbers. First I am...

The cosmological "Big Rip" and a shrinking Hubble radius Doesn't an expanding universe forecast such outcomes as a faded and unobservable cmb, galaxies moving away from each other greater than the speed of light, a gravitationally unbound solar system, and eventually an observable universe...

I've long wondered about an assumption that we have today and I've never found a direct answer to my question. Presently we can observe that there is a direct proportionality between an object distance and the factor by which its light is redshifted. We deduce that this observation implies...

The vacuum-vacuum expectation value in the absence of a source is in general not equal to 1, but exp[-iEt], where E is the energy of the vacuum. For some reason in QFT, we say E=0 (i.e., we normalize Z[0]=1, the generating functional), but we don't need to do this and one can in fact calculate E...

Let us assume a flat FRW metric ds^2=-dt^2+a(t)^2(dx^2+dy^2+dz^2). where t is cosmological time, x,y,z are comoving space coordinates, the speed of light c=1 and a(t_0)=1 at the present cosmological time t_0. Imagine a light beam traveling in the x-direction. It travels on a null geodesic...

Einstein discovered that general covariance allows his GR equation to have just TWO gravitational/geometric constants: Newton G and a curvature constant he called Lambda. So the symmetry of the theory requires us to put both constants into the equation and investigate empirically whether or not...

Since Omega-lambda is very close to Omega-matter, what could it mean if we assume they are exactly equal to each other. Also, let's assume they were always equal since coincident problem is unnatural.

Can the cosmological redshift be interpreted as atomic frequencies increasing by the scale factor as the Universe expands? This explanation seems closer to the truth than the popular idea that a photon's wavelength somehow expands while it travels to us from a distant galaxy. Metric expansion...

Ok, I'm writing up something and I do something I always hate doing... I start arguing myself into a corner. The argument of the night is the expansion of the universe and how it applies to redshifts. Which has brought a series of questions which I can't seem to grasp right now. I) Is...

I have always wondered about how cosmological constant is characterized. You often read the “cosmological constant measured to be ….”.So since it is still a hypothesis, shouldn't the statement read “cosmological constant calculated to be ….” . Or Is it that such semantics does not matter.

A positive value for the cosmological constant was found by the studies of Adam G. Riess et al and Pearlmutter et al In terms of Planck units, and as a natural dimensionless value, the cosmological constant, λ, is on the order of 10−122 or 10−29 g/cm3 The cosmological constant has negative...