92
votes
Accepted
Is there a star over my head?
Summary
There's a 1 in 500 billion chance you're standing under a star outside the Milky Way, a 1 in 3.3 billion chance you're standing under a Milky Way star, and a 1 in 184 thousand chance you're ...
- 848
85
votes
Accepted
Does the Milky Way move through space?
Does the Milky Way move through space?
Yes it does.
I'm very fascinated with space, although I don't have a degree or any formal education, I'm still very in love with everything about it and ...
- 2,523
59
votes
Accepted
Will there be collision between universes?
The Universe is space and time. If there are other universes they can't be "next to" ours, as "next to" is a statement about the relationship between things in space, and the ...
- 116k
54
votes
Accepted
How do scientists know that the distant parts of the universe obey the physical laws exactly as we observe around us?
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 ...
- 116k
41
votes
Does the Milky Way move through space?
Galaxies move through space with velocities of the order of a several 100 km per second; small velocities for small groups (~100 km/s; e.g Carlberg et al. 2000) and large velocities for rich clusters (...
- 37k
38
votes
What would happen to a polished marble statue left in space for a million years?
There are three main space weathering processes that will affect the surface of the marble.
Cosmic rays, high energy particle from the sun and beyond, will hit the surface. This can change the ...
- 116k
34
votes
Are there more stars in the universe than grains of sand in the Earth?
A quick google gave me these (approximate) figures:
7.5 x 1018 grains of sand in all the beaches and deserts of the world
7 x 1022 stars in the observable universe
If these are reasonable estimates, ...
- 1,546
32
votes
Accepted
Are there only $10^{83}$ atoms in the universe?
This is a reasonable estimate for the number of atoms in the observable universe.
It might seem like a small number, compared with the number of atoms in a human only as a result of our brain's ...
- 116k
31
votes
Accepted
Can there be planets, stars and galaxies made of dark matter or antimatter?
Dark matter galaxies are possible but very speculative. On a theoretical level, they are hard to form because dark matter interacts only gravitationally (see Anders Sandberg's answer), which makes it ...
- 3,992
30
votes
Accepted
What exactly is the "paradox" in Olber's Paradox?
Olber's Paradox was created at a time before the idea of a finite universe was accepted. (It was thought of in the 1600's). In order to resolve Olber's Paradox, you have to introduce the idea that ...
- 3,156
28
votes
Can there be planets, stars and galaxies made of dark matter or antimatter?
Probably not. Dark matter should really be called "transparent matter" since it does not interact with light. This has an important consequence: it is hard for dark matter - whatever it is - ...
- 14k
27
votes
Accepted
Does the recent news of "ten times more galaxies" imply that there is correspondingly less dark matter?
All Conselice et al. (2016) appear to suggest is that when you look at something like the Hubble deep field, there are many faint (and presumably low mass) galaxies that are not seen. This has ...
- 146k
21
votes
Accepted
Will we start seeing galaxies disappear due to Universe expansion?
It is a common misconception that galaxies receding faster than light cannot be observed. There are two versions of this misconception:
Galaxies that are now receding faster than light cannot be seen....
- 37k
21
votes
Accepted
If the universe were infinite, how would it look through a telescope?
If the universe were infinite in space, but finite in time, we could see back no further than a light-travel time of the age of the universe. — This is very possibly reality. We can see back to ...
- 116k
20
votes
Accepted
What equation tells you how far in space you can go from a point and return?
Preparation
Let $a(t)$ be the cosmic expansion factor. Let $x$ be a comoving coordinate, so two objects (like galaxies) that are separating solely due to the expansion of the universe have a constant ...
- 4,260
19
votes
Accepted
What is the most dense object in the universe?
Let us define this as the largest observable density of a stable object, in order to exclude black holes which may have a very large (infinite) density at their centers or objects collapsing towards ...
- 146k
18
votes
How can redshifted light be detected?
In a redshift (whether that be caused by relative motion, gravitation or cosmological expansion), all wavelengths are increased by the same factor.
Redshift is determined by identifying features in a ...
- 146k
17
votes
How can 'HD 140283' be older than the universe?
Later estimates shows that the star could be as old as 14.5 billion years (± 0.8 billion years), which is still it older than the universe's calculated age of about 13.8 billion years, an obvious ...
- 33.5k
17
votes
Accepted
Why black holes are extremely cold?
Under General Relativity (GR) alone, a Black Hole's (BH's) event horizon is a point of no return -- anything that passes through the event horizon is lost and gone forever, and nothing comes out. ...
- 7,560
17
votes
What are the stages in the life of a universe?
Yes there are. They are mainly based on what dominates the energy density of the universe at the time and they are known as epochs.
Thus we have the inflationary epoch in the first tiny fraction ($\...
- 146k
17
votes
How do we know we're not getting bigger?
Matter contraction: If everything got twice as big then you are right that rulers etc. would not be able to measure it. However, some physical constants are expressed (partly) in meters, and it would ...
- 271
16
votes
Accepted
Latest cosmological parameters
Cosmological parameters are measured in a variety of ways, and their values will depend on which measurements you trust the most. The paper you link to (Planck Collaboration et al. 2016) with the 2015 ...
- 37k
16
votes
Is there a star over my head?
In short: no one knows for sure, but currently it looks that the probability is 1.
Longer: On our current understanding, the Universe is probably infinite in space. This depends on the recent WMAP ...
- 3,151
16
votes
What equation tells you how far in space you can go from a point and return?
sten's answer is excellent and beautifully analytical; I just wanted to illustrate the journey and show that neglecting matter (and radiation) is a good approximation.
Spacetime diagram
To do this, I ...
- 37k
15
votes
Is the earth bombarded equally in all directions by neutrinos?
There are only two types of neutrino source that are "bright" enough to be reliably detected. The sun and nearby supernovae.
The source of solar neutrinos is nuclear fusion, which is also the source ...
- 116k
15
votes
Accepted
Is the age of the universe relative to an observer's location in that universe?
You are labouring under the misapprehension that how far we can see directly gives the age of the universe. Whilst it is true that the oldest light we can see was emitted some 13.7 billion years ago, ...
- 146k
15
votes
Are there only $10^{83}$ atoms in the universe?
I similarly thought that the difference in mass between a proton and an electron was minuscule. I think the proton is like 3x10^(-27) kg and electron is 3x10^(-31) kg.
But the way you think about it ...
- 251
15
votes
How do we know we're not getting bigger?
We do not know whether or not we live in a simulation in which our capricious simulation overlords have conspired to hide evidence that we are growing larger. Discounting that possibility, science ...
- 33.5k
15
votes
Accepted
Why speed of light is considered to be the fastest?
The recession rates relevant to Hubble's law are not relative velocities, so there's no reason to compare them to the speed of light.
They can be defined as follows. Conceptually, imagine a chain of ...
- 4,260
Only top scored, non community-wiki answers of a minimum length are eligible