# Tag Info

18

A physicist, me for example, identifies events by choosing a set of coordinates. For example I have a clock that I use to record time and a ruler that I can choose to measure distance. This allows me to set up some coordinates $(t, x, y, z)$ so I can assign every event to some point in my coordinate system. If I received a laser pulse from Mars at 16:05 ...

7

Yes there is. The solution to the Friedmann equation in a flat universe with a cosmological constant is $$H^2 = \frac{8\pi G}{3}\rho + \frac{\Lambda}{3},$$ Thus, as the universe expands and the relative importance of gravitating matter, characterised by density $\rho$, decreases, then $\Lambda \simeq 3H^2$. We are already (just) in a dark energy dominated ...

4

Although not a complete answer, one place to start is with the coldest naturally occurring place in the universe, which is the Boomerang Nebula, a planetary nebula that is around 1 K. As best as I can tell, this cooled below the CMB temperature simply by adiabatic expansion, and is insulated in its interior from CMB heating. Is this a feasible way to get to ...

4

Neutrinos are notoriously difficult to detect . Have a look at this review paper : A core-collapse supernova will produce an enormous burst of neutrinos of all flavors in the few-tens-of-MeV range. Measurement of the flavor, time and energy structure of a nearby core-collapse neutrino burst will yield answers to many physics and astrophysics questions. ...

3

Let's do a Back-of-the-Envelope calculation. It is typical for large-scale neutrino calorimeters (I have KamLAND specifically in mind because I worked on the project and know the detector reasonably well) to have an energy-scale uncertainty of a couple of percent at a few MeV energy. That's a systematic, and will effect all results more or less equally. ...

3

It would not give the impression the universe was expanding. The expansion of the universe is related to the Ricci tensor and scalar. For a black hole of the type you describe the Ricci tensor and scalar are both zero. If you mark out any volume of space and watch it then its volume will not increase with time. However in the black hole geometry the Weyl ...

3

The universe, on a larger scale, is of course expanding, but it doesn't really mean that none of the two galaxies should be allowed to run into each other. Because expansion of the universe is not the only factor that will decide the relative separation between the galaxies. The other major factor which affect the relative separation between the galaxies is ...

3

The universe, as far as we can see it, is fairly uniform, with no edge and no favored direction (Axis of Evil being a possible counterexample, but not here relevant). So, each galaxy has galaxies on all sides of it, all radiating energy towards it. The reverse is also true: each galaxy radiates energy in all directions. The net result is that the radiation ...

3

The reason that there is a CMB is because of the big bang. The photons from the very beginning of our universe has spread (almost)uniformly throughout the universe to give rise to a general noise which we call background. Now to answer your question, as the universe expanded after big bang the photons got redshifted and their energy decreased. Now the avg ...

3

At the classical framework , i.e. no General relativity and astrophysical observations of the 18th century , this is a valid question. When talking of a "Universe" one must have a model , and the model depends on the state of physics knowledge at the time of the model. The second law states that entropy always increases or stays the same. One can make a ...

2

The elements that make up the bulk of the Earth were part of the presolar nebula. A similar (though not identical) mixture of elements is found in meteoritic material, which is thought to more accurately represent the mean abundances of that nebula (minus the volatiles) and indeed also agrees with the abundance patterns in the Sun. There are grains of ...

2

I think it has to do with their relative intensities and the high density of food compared to the low density of the universe, This is correct. The microwave ovens are not working with a black body radiation curve with an average of 3K , nor is the heating effect a thermal balance between two black bodies: food and microwave. Bodies in space away from ...

2

The values of individual entries in the metric tensor depend on the coordinate system you choose. In the case of the FLRW metric there is a natural choice of coordinates called the comoving coordinates. In particular the comoving time has a very simple interpretation because it is equal to the proper time of a stationary observer, which obviously means ...

2

Determining what is happening "right now" on a planet 63 light years away is exactly the same as determining what will be happening 63 years in the future on Earth. Both problems are, technically speaking, impossible. We can't know exactly what the future holds 63 years from now, as things may change in surprising ways, just as we can't know what is ...

2

The infalling observer can 'see' whatever events are in its past light cone. The past lightcone of the infalling observer at the point of intersection with the horizon does not enclose the entire exterior region. In fact, no point on the infalling trajectory does, even at the singularity. Therefore the infalling observer unambiguously does not see the "end ...

2

Questions like this are complicated because you have to be clear what you mean by time. The simplest definition is that time is what is shown on a clock, so if I was holding some hypothetical clock that had been reset to zero at the Big Bang my clock would currently be showing 13.799 billion years i.e. the age of the universe. The question then becomes what ...

2

The spatial resolution of a telescope is going to be limited in what it can resolve by something called the diffraction limit. Basically, light can only be focused so much by a lens given its initial starting size and the focal length of the lens. Its useful to think about this in terms of angular resolution for the case of telescopes, and the minimum ...

2

There is another big problem with ultra-small luminosities: due to the small initial light + 1/r² decrease, it might be that only a few photons per hour sent by your target planet reach the diameter of Earth (better be in your telescope ! ). At very small luminosity you have to remind that light is not continuous and made of photons. And way before the ...

2

A space train leaves Mars at 14:00pm and arrives on Earth at 19:45. The train moves at 0.001C and has 40Km of length. How long will it take for the whole train to arrive on Earth? - Disregard re-entry and friction. Nobody on Earth will say the train is leaving mars now. Same thing with the light, just it moves faster and is smaller than the train above. ...

2

Definition 1. A spacetime is said to be spatially homogeneous if there is a one-parameter family of spacelike hypersurfaces $\Sigma_t$ foliating the spacetime such that for each $t$ and for any points $p,q\in\Sigma_t$ there is an isometry of the spacetime metric $g$ which takes $p$ to $q$. Definition 2. A spacetime is said to be isotropic if at each point ...

2

The gravitational red shift is only significant for black holes – where the coefficient may grow arbitrarily large in the vicinity of the horizon – and the neutron stars – where the frequency drops to something comparable to 50%. For all other celestial objects, the red shift is much smaller than one. And only planets and white dwarfs are objects for which ...

1

A Galaxy cluster could have $10^{14}$ solar masses within a radius of 5 Mpc. In this case $GM/Rc^2 \sim 10^{-6}$, equivalent to a velocity shift of less than 1 km/s. Our own Milky Way has a mass of around $10^{12}$ solar masses within 100 kpc. This gives a gravitational redshift of about 100 m/s. These are completely negligible compared to cosmological ...

1

With your description, if no radiation comes back from the surface of the black hole, the temperature should be the vacuum classical temperature, 0 Kelvin. BUT Hawking predicted a radiation coming out from quantum mechanical interactions with the vacuum at the limits of the event horizon. Hawking showed that quantum effects allow black holes to emit ...

1

I would guess that you're thinking of the brane world ideas. If so, the branes you're describing are not eleven dimensional. They are surfaces of dimension lower than eleven embedded in the eleven dimensional space. For example in this context example our universe would be a four dimensional brane.

1

The Big Bang is a mathematical model of how the observable universe evolved , based on fitting astrophysical observations. Like all models it has its region of validity. When I read cosmology fifty years ago, the model included a singularity at the origin, because that is what the mathematical functions of the General Relativity solutions showed. The model ...

1

The precise shape and medium details of our galaxy are very unknown: we are at the worst possible place to figure them, even if the first survey try to figure roughly our spirals and (vaguely) sub-spirals. So we won't recognise our galaxy. But we might recognise it's neighborhood (close as you say, or far), and deduced it's us at the middle. Still if you ...

1

Your question assumes that the universe started out as a point at the Big Bang and then expanded outwards, however this is not the case. Have a look at my answer to Did the Big Bang happen at a point? for more on this. However your main point remains, that is shouldn't gravity be slowing the expansion, and indeed it did until a few billion years ago. The ...

1

$p=\frac{1}{3}\rho$ is the well-known equation of state of a photon gas. It may be derived by looking at the ultra-relativistic limit of the energy momentum tensor for a bunch of particles.$^1$ $p=-\rho$ follows from the fact that the energy momentum tensor of $\Lambda$-style dark energy is proportional to the metric. Thus, at a point and in the proper ...

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