# Tag Info

34

To avoid more complex definitions of temperature (which do not require matter), you could say instead that "an object in interstellar space would be in thermal equilibrium with its environment when it is at a temperature near $3K$." The matter nearby is too diffuse to affect the temperature much. Instead, it is thermal equilibrium mostly due to radiation. ...

29

Temperature in a gas is the average kinetic energy per particle. As an intrinsic property its value is entirely decoupled from how much stuff has the property. Whether there are 100 particles per cubic centimeter or only 1 particle per cubic meter, the temperature can be anything. The coldest parts of the ISM are about 3 K, and getting colder than this is ...

17

When you're trying to understand the mechanics of a system it's usually convenient to choose coordinates that reflect the symmetry of the system. The solar system is roughly centrally symmetric because the Sun is by far the largest mass in it, and the coordinates that reflect this symmetry are polar coordinates with the Sun at the centre. For example in ...

14

1. If the universe is expanding, what is it expanding into? The universe isn't expanding into anything. Space-time isn't curving into a higher-dimensional space. So what do we mean by "curved" and "expanding", words usually having a meaning only for objects in space? The answer is it is just an analogy. Mathematicians have found properties of space an ant ...

11

It's all about the context in which you want to analyze particular issue. If you are studying the solar system, the most suitable, would be to consider the sun as the center of the system. If you are studying the Milky Way, the sun is not a good reference point, you should take the center of the galaxy. Similarly, to locate the stars from an observer on ...

10

Too long to be a comment, this is an extension to Chris's answer. Suppose a macroscopic object, a thermometer, for example, was placed in that hot intracluster medium (ICM) Chris mentioned in his answer. Even though that thermometer is surrounded by this hot gas, the thermometer will not get hot. It will instead cool to a tiny bit above the cosmic microwave ...

8

why not relative to the Earth? Scientists do express things relative to the Earth, where that makes sense. I couldn't imagine trying to forecast the weather or model the global circulation of the Earth's atmosphere from the perspective of a non-rotating frame with it's origin at the solar system barycenter. Astronomers, at least those dealing with ...

7

The answer is yes. The de Broglie wavelengths of freely propagating particles (i.e. forget the influence of interactions and gravity perturbations, just consider the Universe as a whole) are redshifted by the expansion of the universe. Another way of saying this is that their peculiar momenta with respect to a co-moving local volume decrease as the inverse ...

6

The simple answer is that your cousin could be correct. If his theory is that: the scale of the sphere is far larger than the observable universe there's no way to detect the 4th (spatial) dimension then no experiment we can do could prove him wrong. But then there's no experiment that we can do that could prove him right either, so as theories go it ...

4

A reference frame at rest with the Sun is, with a good approximation, an inertial system (much better than one at rest with our planet or other bodies in the Solar system, essentially in view of the hugely larger mass of the Sun). Physics in inertial reference frames has the simplest form. For instance the motion of planets around the Sun is described along ...

3

Your 12-year-old cousin might be correct; it isn't yet known for sure. However, some existing experiments are pointing in the direction of your cousin being wrong. What you're calling a "4D sphere" and a "3D sphere", a mathematician would call a "3-sphere" or a "2-sphere", respectively, because mathematically an "$n$-sphere" means something that's ...

3

Although there are suggestions that a black hole could lead into another expanding universe you should regard these as highly speculative. Although the metrics can be stitched together it requires a contribution from currently hypothetical quantum gravity effects. Until we have a working theory of quantum gravity there is no way to comment on how likely the ...

3

How do we know that space expanded faster than a speed of light in inflation? Let us start from the beginning, on the reason that the Big Bang theory was proposed as a model for the universe. The reason was the observations that all clusters of galaxies were receding from each other. This is what happens from an explosion at the center, in three ...

2

Inflation does not violate any local speed of light physics and there is no global prohibition in general relativity against spacetime points that are moving away from each other faster than the speed of light. Such spacetime points are simply not causally connected, i.e. there is no physical way to communicate between them (since light signals from one can ...

2

The argument is sound given a few oft-omitted (but not too unreasonable) assumptions. Here is one way it can be formulated. Consider a volume $V$. Suppose it has a (possibly infinite) set of possible configurations; call this set of states $S$. Suppose we are interested in a particular configuration, $c \in S$, to within a certain tolerance. Let \$C ...

2

You ask: Is it that space is not expanding within the smaller structures or is space expanding through these structures? where I've highlighted what I think is the key issue. The phrase space is expanding is a convenient metaphor to describe the expansion of the universe, but it is only a metaphor and taking it too literally can lead to confusion. It ...

2

Even if we could assume an empty closed universe, the answer depends on both, the size of the black hole and the size of the universe. The smaller the black hole the faster it evaporates, and the larger the universe the longer it takes the radiation to come back, so the black hole can be evaporated before the radiation comes back.

1

BECAUSE WE MEASURE WITH ATOMS (or atomic properties) a change in the measured value can be either a true change of the observed quantity or a change in the properties of the ruler (the atoms). I'll revert the paradigm the space expands to this one: the atoms are shrinking THEN I have simple answers : If the universe is expanding, what is it ...

1

The argument rests on the assumed validity of Ergodic theory (see http://en.wikipedia.org/wiki/Ergodic_theory). Quoting it "A central concern of ergodic theory is the behavior of a dynamical system when it is allowed to run for a long time. The first result in this direction is the Poincaré recurrence theorem, which claims that almost all points in any ...

1

There's an old theory called "tired light" where the momentum is lost due to waves hands some other reason, but as far as I'm aware this has been pretty much discounted these days. The background behind the current-best-theory is this: When you look at light from a star it's not a smooth spectrum, it has a series of dark lines in it, an "emission ...

1

My answer to your question is an absolute "no". First of all Black holes do not implode, that would suggest them blowing up in some way, they can grow but to say implode is null.

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