# Tag Info

51

We don't know in general but to the extent we can measure, the laws seem to be the same, even if conditions are not. For example radioactive decay: We know how fast various elements decay, and we can observe the results of radioactive decay in distant supernovae. The conclusion is that, for at least some elements, the rate of radioactive decay is the same ...

9

It depends a bit on what you means by "far away" and "the same", but: Galaxy formation Galaxies form from collapsing and colliding clouds of gas and dark matter in the early Universe. The first structures began to form a few hundred million years after the Big Bang, with masses of the order of $10^5$ Solar masses (e.g. Mo et al. 2010). As ...

8

The age of the universe is not calculated based on the size of the visible universe. The age of the universe is being calculated based on the fact that the laws of nature have no direction. This means that you can use the laws of nature to predict future behavior, but also assume previous behavior. Based on calculating backwards with the laws of nature, for ...

8

That diagram does not depict the entire universe. At most, it depicts the history of what is now our observable universe (specifically, a 2D slice through it), with us at the center only because we're observing it. Someone at the furthest reaches of that portion of the universe would see us at the furthest reaches of their observable universe, and themselves ...

7

See also: Do the laws of physics work everywhere in the universe? Noether's theorem, in the context of this question, states that: If the laws of physics do not vary with position, then linear momentum is conserved (and vice versa). Therefore if we observe conservation of momentum (which we do with exquisite precision) then we do not expect the laws of ...

5

In 2-dimensions it is easy to compare the rotation direction of two systems. They can either rotate clockwise or counter-clockwise. In three dimensions, spin direction can better be described by the pointing direction of the spin axis or angular momentum vector (normal to the galactic plane). Imagine if all the galaxies rotated in the same direction as the ...

5

There are many extremely widespread misconceptions about cosmology. One is the idea that there's some importance to cosmological recession speeds larger than $c$. In reality, recessional speeds are defined in a somewhat peculiar way and the value $c$ has no significance in them. It's not a limiting speed, and it's not in any useful sense "the speed of ...

5

The CMB is visible at a distance of 13.8 billion light years in all directions from Earth, leading scientists to determine that this is the true age of the Universe. This is wrong in a few ways. First, we do have good reason to think that the CMB was produced around 13.8 billion years ago, but that doesn't mean it's 13.8 billion light years away. The light ...

5

There are several different quantities of this sort that you can define, and the definitions are fairly confusing. Hopefully the following diagram will make things clearer. Z <- future infinity / \ / \ / \ D C B A B C D <- now ...

4

You're completely correct! The farthest we can see (in principle, not in practice) is called the particle horizon. Currently, the distance to the particle horizon is $d_\mathrm{P} \simeq 46\,\mathrm{Glyr}$, but as time goes on, light from more and more distant regions will reach us. If the Universe contains only "regular stuff" such as normal ...

4

The part of the universe that we can see is a cone in spacetime (our past light cone) and there are young and old parts of the universe both inside and outside the cone. Here's a picture from Ned Wright's cosmology tutorial that shows what's going on. We're at the top center. The red lines are (a slice through) our past light cone. Ignore the wavy $\phi(x)$ ...

4

With any given star, Jupiter has 3 or 4 triple conjunctions per century, with either 12 or 71 years in between. These occur with Regulus in 1873, 1885, 1956, 1968, 2039, 2051, 2063, 2134, and 2146. Jupiter takes 11.9 years to orbit the Sun, advancing at a fairly steady ~30° per year. From Earth, Jupiter appears to move ~40° forward and ~10° ...

3

No extraterrestrial life has ever been found and we only know of one creature that has formed a civilisation: Homo sapiens. And we have not yet reached type I. So we know nothing from observations about civilisations that are beyond our own. Kardashev wanted to have a way of thinking that didn't put "humans" at the top, so he described types I, II ...

2

Orbital eccentricities and inclinations are damped to zero during the protoplanetary disc phase. This is because the gaseous parts of the protoplanetary disc are a great sink for excess angular momentum. After the gas disc dissipates, the formed planets start to interact dynamically as they do not necessarily form in stable configurations. This can lead to ...

2

I think the main reason the answer doesn't come out correct is that the simple Hubble's law equation: $v = dH_{0}$ Is only applicable in the local universe (Hence $H_{0}$, the value of the Hubble constant today). The rate of expansion of the Universe (the Hubble parameter) is determined by the composition of its energy density. Today, the composition is ...

2

According to Wikipedia, GN-z11 has a redshift of 11.09, which, according to Ned Wright's cosmology calculator, corresponds to a light travel time of about 13 billion years and a comoving radial distance of about 32 billion light years. The comoving radial distance is the distance measure with respect to which the diameter of the observable universe is about ...

1

Luminosity function Your approach is in principle correct, but would give the number density $n$ of Lyman $\alpha$ emitters (LAEs) in a "luminosity volume". Usually, the comoving volume is used, since you can then more easily compare densities at different redshifts $z$. If $n$ changes with $z$, you then know that it's not just because of the ...

1

the expansion of the universe is relative to where it is being observed from. Much like dots on a balloon. Its easy to point to the center of the balloon (like what the picture shows) because it is only a 3d object. But for higher dimensions we don't have the spacial awareness to understand where than center of the universe is (if there even is one)

1

Your question isn’t well stated and I think it shows an unclear understanding about cosmic horizons, the consequences of the speed of light and in general and the notion that the universe has no center. I could suggest a few videos. “If objects created after the CMB are outside our observable universe, why is the CMB our temporal edge” makes no sense. First ...

1

Indeed this is correct. Photons can travel distances greater than the radius of the observable universe without interacting with matter. This has been true ever since the period know as "recombination" when electrons became bound to hydrogen and helium nuclei. Prior to that time there were lots of free electrons that could interact with light. ...

1

Your intuition is correct - a moving source emitting wavefronts periodically will be closer to the previously emitted wave in the direction of motion, and farther from the previously emitted wave in the opposite direction - see the simulation here. You are also correct that the size of the effect depends on the speed of the observer relative to the speed of ...

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