# Tag Info

29

Measuring $w$ is actually what I do for a living. The current best measurements put $w$ at $-1$ but with an uncertainty of $5\%$, so there's a little room for $w \ne -1$ models, but it's not big and getting smaller all the time. Indeed, we'd all be thrilled if, as measurements got more precise, $w \ne -1$ turns out to be the case because the $\Lambda$CDM ...

24

The simple answer is that no, time is not expanding or contracting. The complicated answer is that when we're describing the universe we start with the assumption that time isn't expanding or contracting. That is, we choose our coordinate system to make the time dimension non-changing. You don't say whether you're at school or college or whatever, but I'm ...

24

The accelerated expansion of the universe is not direct evidence for dark energy, i.e. a perfect fluid contribution to the stress-energy tensor with $w = -1$. Dark energy is just by far the simplest thing that fits the data well. It's simple to cosmologists because they are used to dealing with matter in the form of perfect fluids, and dark energy is just ...

24

These aspects of astronomy and cosmology are indeed very interesting and very significant, but don't allow the names to get in the way of your understanding. Dark matter is a form of matter made (most likely) of particles which don't interact very much with the matter we are more familiar with (i.e. protons, neutrons, electrons etc.). The evidence for it has ...

23

The title and the text actually ask two different questions. While Kyle Oman and Thriveth answer the title excellently, I'll address the question in the text which asks "Why did the Universe expand in the first place, before dark energy (DE) started to dominate". The answer to this is inflation (we think). The first fraction of a second after the creation ...

21

Let's start partway through the expansion of the Universe in the matter dominated epoch. At this time the energy density is dominated by matter, but the dark energy and radiation components are still present, just relatively small. The Universe is expanding, but the expansion is gradually slowing down. As the Universe expands, the density of matter scales ...

15

If dark energy would consist of particles, it would dilute with the growing radius of the universe to the third power, since the total number of particles would stay the same while the volume increases. What observations found was that dark energy rather behaves like a constant which does not thin out, that's why it is also known as the cosmological constant....

15

Dark energy as expressed by the cosmological constant is, as the name implies, a constant of space. Therefore, in a cup of coffee, we get, for some static observer $t$, and a spacelike hypersurface $\Sigma$ (I'm assuming that in our universe, there exists a neighbourhood that can be foliated in spacelike hypersurfaces large enough to accommodate a coffee cup)...

14

A positive cosmological constant leads to positive scalar curvature by definition. Just trace over the Einstein equation and you end up with $$R = 4\Lambda - 8\pi T$$ which is just $$R = 4\Lambda > 0$$ in vacuum. The implicit, but more interesting questions are probably the following ones: Why can we interpret the cosmological constant as dark ...

13

It's not clear if you're asking for details of how the CMB power spectrum is analysed, or whether it's a general question about how this sort of measurement is made. I'll assume the latter - hopefully this will be of interest to others even if it isn't what you intended. There is a good discussion of the procedure in the Wikipedia article on the Lambda-CDM ...

13

I am an experimental physicists, and the model in the first paper has not reached the level of experimental predictions, for LHC results. In fact except for the link you give the search at the CERN document server gives nothing, and the word "weakton" does not yield discussions or appraisals. So the experimental physics community is overlooking ...

13

There are (at least) four ways in which the dark energy content of the universe influences things we can observe The cosmic microwave background is formed in the early universe when atoms (of hydrogen) first formed and the universe became transparent to the radiation that was within it. There are small fluctuations in the CMB which reflect small differences ...

12

It's not as naive a question as you may think, and the answer is a lot more complicated than you may think. When we're calculating how the universe expands we assume it's isotropic and homogeneous (this just means on average it's the same everywhere) and it has a scale factor that is normally written as $a(t)$. The scale factor tells us how much the ...

12

Dark energy is an unknown or unattributed form of energy that is separate and distinct from the other forms of energy. It is not anti-engery. It is dark energy. Anti-energy (were such a thing to exist) would annihilate any form of energy. Dark energy is called "dark" because we aren't exactly sure what it really is or what causes it. The most abundant forms ...

12

I'll try and briefly run through some points (without mathematical detail) to see if this clears up any of your questions • You seem to have misunderstood the 'blunder' part: the 'blunder' wasn't removing the cosmological constant, but adding it to his equations (in an ad-hoc way, at the time) in the first place. • Today, dark energy isn't 'different' from ...

11

Yes. Modified gravity theories are credible. Daniel Grumiller and Jerry Schirmer have pointed out some of the case against it, but there are deep, potentially intractable problems with a dark matter particle approach as well. Also, the weight of the evidence as shifted as astronomers, particle physicists and theorists have provided us with more relevant ...

10

My area of research is (related to) large scale structure and substructure of dark matter. Not sure if you're interested more in the question of "what is dark matter" (from more of a particle physics point of view) or "what do we know about how dark matter is distributed and its gravitational interactions" (a more astronomy point of view). This answer leans ...

10

I though I would discuss the transition from radiation to matter dominated phases and from there to the dark energy phase. A fair amount of this can be discussed with just Newtonian mechanics. General relativity changes this by some subtle means, but as a coarse grained view, to borrow a stat mechanics term, Newtonian mechanics captures a lot of this. We ...

10

Why are we trying to fit GR in QFT when there's a chance that GR is incomplete? There's not just a chance that GR is incomplete. It is certainly incomplete: it can't account for quantum phenomena. That's why physicists are not trying to fit GR in QFT. Instead, they're trying to find something new that reproduces the successful predictions of GR while also ...

9

We indeed expect that the universe expands on the basis of the Big Bang theory. Hence by looking at higher redshift, i.e. further in time, you should expect objects to recede faster. This is reflected in the linear relationshift that first measured by Hubble $$v=H_0D,$$ where $v$ is the recession speed, $D$ the distance to the object and $H_0$ the Hubble ...

9

Short answer: The ratios have changed over time... drastically. This is a consequence of the expansion of our universe. Initially (and by that I mean after the conjectured inflationary epoch, which I will not consider here), radiation dominated all other forms of energy by far. However, as the universe expands---as measured by the increase of the ''scale ...

9

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 ...

9

The short version: The amount of matter in the Universe is fixed, so as the Universe expands, matter density will drop because the same amount of matter will be spread out on more space. Dark Energy, on the other hand, is (by definition) constant or almost constant in density. This means that no matter how dilute the Dark Energy is, if it waits long enough, ...

9

In terms of direct numbers at present, dark energy comprises about 70% of all of the energy in the universe. Radiation, on the other hand, makes up less than 0.005% of the energy in the universe. It's such a small fraction that it's less than the error associated with the values for matter and dark energy. A good way to approximate how the two energies ...

9

On one hand, according to observations, our universe has a positive cosmological constant $\Lambda>0$ associated with dark energy. On the other hand, string theorists have so far not rigorously constructed a single 4D de Sitter stable vacuum with $\Lambda>0$ in string theory [1]. A de Sitter swampland conjecture [2] suggests that they don't exist. See ...

8

Is it because the acceleration is too weak? It is too weak with respect to the four forces we measure. The fact that the four known forces are so much stronger means that agglomerates of particles, up to the scale of galaxies are not internally affected, they keep their structure intact, like the famous raisins in the rising bread. It is only at the level ...

8

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 ...

8

The most distant object that light we emit today can reach in the distant future is at the event horizon $$eH(t) = a(t)\cdot \int_{t}^{t_{max}} \frac{c\cdot \text{d}t'}{a(t')}$$ which is now approximately 17 billion lightyears away, see the future light cone in comoving coordinates which converges to this distance: If the light was emitted at the big ...

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