# Tag Info

## New answers tagged cosmology

0

The scale-factor of the universe changes significantly over that period of time, so you can't calculate distance as simply $d = v \cdot t$. You have to actually integrate over the expanding spacetime metric, i.e. $$s = \int_{z_1}^{z_2} \frac{c_s}{H(z)} dz$$ Where $c_s$ is the speed of sound (and I think that should be $c/\sqrt{3}$ instead of $c/2$, ...

6

We want the Newtonian limit of the Einstein Field equations for nonzero vacuum energy(=cosmological constant). As $\rho_\mathrm{vac}=\Lambda/4\pi G$ is a mass(=energy) density, Poisson equation is $$\Delta\Phi=4\pi G\rho(\boldsymbol r)-\Lambda \tag{1}$$ If we assume spherical symmetry, and point-like source $\rho\sim\delta(\boldsymbol r)$, the ...

3

You get an extra term that increases with r: $$a = -\frac{G\cdot M}{r^2} + j\cdot r$$ with j as the repulsive component.

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Here's another, simpler answer. The BB analyses (like the one above) are typically based on some very modest definition of life, like an unpressurized brain floating in vacuum. If you include all the other equipment necessary for a brain to actually function then the odds are much different. Furthermore, the universe actually doesn't contain much ...

0

In the geometrical optics approximation light ray is represented by a null geodesic. Therefore you only need to find a null geodesic connecting points $(t_0,0,0,0)$ and $(t_1,x,0,0)$ for some $t_1$ (and this condition will determine $t_1$ uniquely). This is probably quite easy to do directly in this case, but in general for investigation of null curves in ...

2

EDIT: My first answer seemed to imply that radiation is at rest in the Cosmic Rest Frame. Radiation is not in rest in any frame. See below. The sentence shouldn't be read as "[velocity of energy] forms", but "velocity of [energy forms]"$^\dagger$. The sentence refers to "energy forms", i.e. the different forms in which energy can manifest itself. These ...

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The Big Bang Theory is a much more general and less specific description of our theory about the origin of the Universe than the $\Lambda{\rm CDM}$ model (by the way, I don't think that the hyphen is written in that acronym). The Big Bang Theory says that the Universe was expanding and the distances between two places where galaxies sit today used to be ...

1

While cnosam's answer is completely correct, I don't know if it really solves your confusion. The key point is that, when a photon is emitted, it knows nothing about the current size of the Universe. It is emitted at a very specific wavelength given by quantum mechanics, not by cosmology. Traveling through expanding space subsequently increases its ...

2

When calculating redshifts, we usually look for signature features in astronomical spectra, usually emission or absorption lines. For example, the universe contains lots of hydrogen. From quantum mechanics, we know that hydrogen has many different energy states which are fixed. This means it can only emit photons with a particular set of wavelengths (these ...

-1

The explosion that you have seen is actually 4 dimensional representation of the universe. if we are representing universe in 4D then big bang had happened at a point and is expanding as a hollow sphere.But in 3D the big bang should have happened in every point to the universe and is expanding to every direction.This interpretation is using Friedman model of ...

1

Firstly, you must know that there are many models for inflation which give different results to your a) and b) questions, and we still don't know which is the right one. I'll try to answer regarding the most accepted and simple models. a) During the period of inflation the distance between two separated points in the Universe increased at least ...

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One of ideas associated with string theory is the ekpyrotic universe. This starts with brane cosmology i.e. the idea that our universe is a four dimensional brane floating around in the ten dimensional string theory spacetime. There will be many such brane worlds and the ekpyrotic idea is that a collision between two branes would appear just like the Big ...

2

No, we will always be able to see farther than the Hubble sphere (in theory). This spacetime diagram — taken from Pulsar's rendering of Davis & Lineweaver (2003)'s Figure 1, in this excellent answer — can help visualize it: Coordinates In this figure, time increases upward, we're the vertical line in the middle, Big Bang is the bottom line, and our ...

2

The vast majority of the star like objects we see in the sky are stars in our own galaxy. Assuming the accelerated expansion is due to a cosmological constant, and assuming the value of the cosmological constant does change (it's currently of order $10^{-52}\,\text{m}^2$) the expansion will never be strong enough to disrupt the Milky Way. So our night sky is ...

2

It's impossible to draw an accurate picture of a 2D hyperbolic surface, because such a surface cannot be embedded into a 3D euclidean space; this is known as Hilbert's Theorem. The saddle surface in the figure is just an approximation, and serves as an illustration that every point on a hyperbolic surface is a saddle point.

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Such measurements have been done, using lasers reflecting off mirrors on the moon. See e.g the paper Progress in Lunar Laser Ranging Tests of Relativistic Gravity (Williams et. al. 2008) which established an effective limit on the expansion at AU scales that is about 80 times smaller than what would be expected if cosmological expansion applied within our ...

0

Is the Metric expansion of space relatively uniform in space? No. In other words, loosely speaking, does expansion happens everywhere, and over a wide range of length scales? No. For example, the Hubble constant (say 70 km/sec per megaparsec) would be about 2.3E-05 m/s at 10 billion km. Neglecting numerous profound experimental ...

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The initial energy to effectively fuel this expansion came from the Big Bang. Energy is not then continuously required to fuel further expansion. The temperature of the universe may decrease due to expansion, but the rate of expansion is accelerating due to currently unobserved dark energy, according to common theory. If the repulsive forces from the dark ...

2

The statement that at the beginning of the universe energy/mass was concentrated in a single region under conditions of extreme temperature and density is usually extrapolated from experimental data on the energy/mass content of the universe and its expansion, which is then analyzed through the classical (i.e. non-quantum) theories of general relativity and ...

0

Then we have alternatives, such as the Big Freeze transitioning into the new Big Bang in the Conformal Cyclic Cosmology Anyway, the short answer is nobody knows. There is too little known about the nature of Dark Energy especially to be able to say what the universe will be like in the far future.

-1

Heat Death and the Big Freeze are the same thing.

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The most straightforward theories of inflation assume there exists some scalar field $\phi$ that permeates the Universe and drives inflation. Over time this scalar field changes, and the rate of change is given by $\dot{\phi}$. There is also some "potential energy" associated with the scalar field, which is given by some function $V(\phi)$. The specific ...

-5

In a static bounded universe, what happens to particles that hit the edge? I venture to say nobody knows for sure. But it's an interesting question, because we have no evidence whatsoever that our universe is infinite or some kind of hypersphere. See this answer to a related subject. The story goes that in the old days, people could not conceive of a ...

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The cosmological redshift of a galaxy is not interpreted as being due to the velocity of that galaxy away from us (the special relativistic interpretation), but rather as being being due the effect of the expanding space on the traveling photon (the general-relativistic interpretation).$^\dagger$ This expansion in turn makes the galaxies recede from us at a ...

0

You say right as Hubble's law state that farthest the galaxy is faster it is receding from us. $v=Hl$. H is Hubble constant. $H(t)=\frac{\dot{a(t)}}{a(t)}$, a(t) is scale factor. Experimentally it is confirmed by observing the radiation from distant object. Due to Doppler's effect observed frequency of light will be different from emitted one and it depends ...

-1

When we talk about universe inflation we should have in mind that the four cosmic forces did not appear yet, because at inflation no elements were formed yet. scientists say that at 0.03 second the inflation reached four light-years, also said that if it had continued at the same rate, it would have been disappeared during the second have of the first ...

1

What is a simple explanation to the fact that the universe is expanding? Show them a star map and point out the galaxies and the fact that it is a projection of three dimensional space to an image. Astronomers and astrophysicists have spent a lot of effort measuring the behavior of the galaxies, their motions relative to each other and to the solar ...

3

A massive object (such as a galaxy) along the line of sight to a distant bright source (such as a quasar) bends the light along its path. If the "lensing" object is massive enough and the geometry is right, the background object can be seen as multiple sources. For instance, here is a galaxy (central point) and four images of a single quasar: For a ...

3

This is an active (hot?) topic of research, in fact I attended a workshop on the subject just last week. In brief, no one has found a dark matter (DM) halo yet that does not host a galaxy, though we would very very much like to! The first reason it's so difficult to find a DM halo that does not have a galaxy is that a common working definition of a galaxy ...

0

This paper explains the importance of physical mechanisms existing which can function as clocks at various eras. Your question is not simple Abstract We provide a discussion of some main ideas in our project about the physical foundation of the time concept in cosmology. It is standard to point to the Planck scale (located at ∼ 10−43 seconds after ...

2

Yes, it can. Curvature (whatever measure for it you use, Riemann tensor, Ricci tensor, Ricci scalar, you name it) is a function of spacetime, and hence of time.

0

Well, the observations of the acceleration of the universe's expansion is perhaps the strongest argument against the Big Crunch argument. Nonetheless though, it is by no means a disproven theory- we cannot very accurately predict the future until we understand the 95% of the universe that isn't baryonic matter, the 95% that current theories have as the ...

2

Until dark energy (and dark matter) are properly understood, it is impossible to be certain of the future fate of the universe. The concordance $\Lambda$CDM model, deduced from observations of distant supernovae, from the cosmic microwave background and from baryon acoustic oscillations suggest that the expansion of the universe is accelerating and that ...

0

I am not an expert to give the most proper answer but according to Hubble's law , the most distant galaxies are moving away from us faster than the speed of light, due to space expansion. This fact makes me think that also the gravitational effect of these galaxies is lost forever, and this looks to be the future of our universe. Even if a 'big crunch' ...

1

This is a very tough question, since you do not make any further assumptions about the force - most importantly its strength, sign, variation with distance and objects it acts upon. In general, yes, the universe would probably look absolutely different and we would not be here to ask this question. Somebody or something absolutely else might. Our ...

0

When describing the formation of a black hole (or the merger of two equally-sized black holes) in a Newtonian gravity framework, Planck-scale gravitational forces of the order $c^4/G$ enter into the description. This is independent of the mass of the black hole. Such can easily be seen by modeling an infalling spherical shell of dust with mass $M$ under the ...

4

Let's analyse the evolution of the curvature in the $\Lambda\text{CDM}$ model. If $\rho_R$, $\rho_M$, and $\rho_\Lambda$ are the densities of radiation, matter and dark energy, and $$\rho_c = \frac{3H^2}{8\pi G}$$ is the critical density, then we can define  \Omega_{R} = \frac{\rho_{R}}{\rho_{c}},\quad \Omega_{M} = \frac{\rho_{M}}{\rho_{c}},\quad ...

0

what is dark matter ??? We know how much dark energy there is because we know how it affects the Universe's expansion. Other than that, it is a complete mystery. But it is an important mystery. It turns out that roughly 68% of the Universe is dark energy. Dark matter makes up about 27%. The rest - everything on Earth, everything ...

-1

Planck's constant is a force associated with each cycle of a photon. For instance a photon with the 500 nm wavelength actually oscillates at a frequency more than 600 trillion times per second. Each time it oscillates, Planck's Constant is applied.

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Throughout the question I will use $p(T_1)$ and $p(T_2)$ to denote the 4-momentum of the baseball at times $T_1$ and $T_2$, $\mathbf{v}_1$ and $\mathbf{v}_2$ to represent the spatial component of its physical velocity, and $a(T_1)$ and $a(T_2)$ to represent the scale factor of the Universe at these times. The homogeneity and isotropy of the Universe mean ...

0

The wikipedia article about the holographic principle does not suggest, that this is well-accepted and founded theory :) The ideas seem to be somehow based on the concept of the Bekenstein-Hawking entropy for a black hole. This is supposed to be the entropy of a black hole from outside (according to the no-hair-theorem the black hole has only three ...

-9

The maximal possible force in universe is something that scientists missed to consider, and if they would consider it, then everything what they have done so far would not be worth a dime, ranging from "point-like particles" and up to the "Big Bang". Point-like particle (i.e. photon), or any other imaginary (invented) structure (i.e. strings (line-like ...

2

A rotating reference frame is not an inertial reference frame: In the rotating frame, objects accelerate even though there are no forces acting on them. In your example, you can in fact determine easily whether you are rotating or the universe is rotating around you. In the first case there is artificial gravity on the ship, and in the second case there is ...

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