# Tag Info

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 ...
• 838
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 ...
• 2,497
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 ...
• 104k
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 ...
• 104k

### 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 (...
• 35.6k

### 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 ...
• 104k

### 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, ...
• 1,536
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 ...
• 104k
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 ...
• 3,594
Accepted

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 ...
• 3,076

### 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 - ...
• 11.4k
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 ...
• 132k
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....
• 35.6k
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 ...
• 1,403
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 ...
• 132k
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. ...
• 7,450

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 ($\... • 132k 16 votes Accepted ### Is there any practical use for astronomy? This question begs the question, does everything need a practical use? The answer is a resounding no. What's the practical use of the Louvre, or of your local neighborhood public park where you enjoy ... • 32.4k 16 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 ... • 32.4k 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 ... • 3,110 16 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 ... • 261 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 ... • 35.6k 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 ... • 104k 15 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 ... • 35.6k 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, ... • 132k 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

### 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 ...
• 32.4k
Accepted

### How does the concept of a universe with no center work?

When we talk about the universe, we are really talking about one of two things: The observable universe, which is everything we can possibly see. The Universe, which is everything that has ever ...
• 7,870