# Tag Info

71

Photons can't have a perspective. If we have a particle with mass, we can imagine taking a frame of reference in which that particle is at rest. We can then see things "from the particle's perspective". But there is no frame of reference in which a photon is at rest. Photons always move at the speed of light in every frame of reference. If I try to set ...

15

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, the stuff that emitted that light is now roughly 46 billion light years away, thanks to expansion of the universe. The universe itself probably extends ...

12

From looking at the Black Hole alone, there is no possibility of determining its age. The state of the Black Hole is fully determined by a few fundamental variables (mass, angular momentum and electric charge). This is the statement of the famous dictum A black hole has no hair. Hawking radiation in special is only dependent on these variables. You may be ...

11

Rob Jeffries gives a good response to this question, but I wanted to go through the basic outline of how the age of the universe is calculated, just so you can see how it works more or less. Be warned though, I'm only giving you the highlights and you'll either have to accept what I'm saying or fill in the blanks yourself. The Friedmann Equation As with ...

11

This is a confusing question - your title mentions GR, but of course the age of the universe is entirely derived as a result of using GR to solve for the dynamics of the expanding universe. The text of your question is talking about time dilation and the effects of special relativity (a subset of GR). Here there is a point to be addressed. It is not a good ...

10

30.4 billion lightyears. The current distance — i.e. the distance that one would measure if we froze the Universe and started laying out measuring rods — is called the proper distance, or physical distance, in astronomy. By definition, it corresponds today to another often-used term in astronomy, namely the comoving distance. While the former increases with ...

9

The material (heavier than helium) that makes up the solar system was made in millions, if not hundreds of millions of stars that lived and died in the ~7 billion years between the formation of the Galaxy and the birth of the Sun. This material has been thoroughly mixed in the interstellar medium and so the heavier elements arise from countless individual ...

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 ...

6

Yes, but not very likely. The closest orbit that does not require constant expenditure of energy to maintain it is the prograde equatorial ISCO. For a Kerr black hole the time dilation factor on this orbit is $$\frac{dt}{d\tau}\approx \frac{2^{4/3}}{\sqrt{3}(1-a/M)^{1/3}},$$ which at the astrophysically likely Thorne limit $a = 0.998M$ gives a dilation of ...

6

There is a quantity in relativity of $s^2$ which is defined as $t^2-x^2$, where $t$ is the difference in time between two events, and $x$ is the difference in position (measured in units such that $c=1$). If $s^2$ is positive, then the square root of it is the proper time. If it's negative, then the square root of $-s^2$ is the proper distance. (BTW, proper ...

5

In the comments of the other answer, the question came up whether decay of the orbit would limit the time amount of time dilation. The answer is of course yes. But by how much? This question can be answered using some modern results for the modelling extreme mass -ratio binaries in the limit of extremal spins. The answer will depend crucially on ratio of ...

5

A type 1a supernova forms when a white dwarf grows through accretion to a certain size, at which it becomes unstable. This means that the precursor object is always a white dwarf of mass 1.39 solar masses. As the precursor object is always of the same type and the same size, the supernova is thought to be the same brightness. On the other hand, type II ...

5

And also there are some terms related to black holes, 3D shape, ball etc. Apart from those complexity, my intention is simply to be clarified, why we call our universe flat? And by word flat, what I meant "having a level surface; without raised areas or indentations". Imho in order to accept the answer, you have to realize the contradiction in the ...

5

Nearby galaxies are seen in their old age. Distant galaxies are, on average, the same age as the local ones. But due to the finite speed of light, the farther you look, the younger you observe them. That means that the most distant galaxies are observed in their infancy, and they look much different from the old ones. Galaxy evolution This is one reason to ...

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

First, Terminology: Age is how old something is. Relative age is how old something is when compared to another (older vs younger). Absolute age is putting a number on that age (I am XX years old). Absolute model age is an absolute age that is based on a model, but where the age has not been measured directly (based on this cat's tooth, its absolute model ...

5

Asteroseismology effectively measures the sound speed inside a star by finding the characteristic oscillation frequencies of a star. The sound speed depends on the composition because the pressure at a given temperature depends on the average mass of a particle in a gas. As the star gets older, it turns its hydrogen into helium, changing the composition, the ...

4

At the moment there is basically only one way. That is to associate the planetary-sized object with a cluster of stars or moving group of stars of known age. That's basically it. If the planetary-sized object really can't be associated with another object, then only limits can be placed on its age by comparing it's luminosity to theoretical planet cooling ...

4

One criterion that works well is the presence or absence of smaller craters inside. If the crater is clean, it's probably new. If it's full of smaller craters, it's old. This applies to planets and satellites with no atmosphere, or with a rarefied atmosphere.

3

There isn't really a database as you request. Finding the ages of stars is difficult. Only one star has an accurately known age - the Sun. That comes from radioisotope dating of meteorites. For other stars we must rely on models to a greater or lesser extent and we can only estimate an age if the star has a mass or is in a phase of its evolution where things ...

3

We do not have direct data on the age of any exoplanets, but we can estimate the age of their host stars, around which they presumably formed at the same time. These methods are uncertain and may have systematic biases but gives at least some information. If we use the Wikipedia page and plot the ages (again, both old data and now extra selection biases!) ...

3

Finding the best-fitting isochrone, a.k.a. isochrone fitting, is a standard approach to determine the age of globular clusters. This problem can be solved with a least-square method, where the data to be fitted are the points on the color-magnitude diagram and the fitting curve is the isochrone. Since there are many ways to apply this method, I will first ...

3

Flat here means "has Euklidean geometry" (on large scales), as explained by Helen's answer. Observationally, the universe appears flat, i.e. any deviations are within the uncertainties. Of course, if the universe is infinite, no statements on the whole universe can ever be made, but only on our local observable patch of it.

3

@ProfRob has already given a good answer to the question, but I wanted to add just a little more detail addressing the observational data side. As Rob already stated, the fundamental data often come (these days) from satellites like Kepler or TESS, which can measure a star's brightness with very high precision, over an uninterrupted period of many days. So ...

3

Of the order 10000–100000 years. As an example of an old Supernova remnant, you can see the Vela SNR, estimated at 11,000–12,300 years old. Nasa describes the evolution of a remnant Initially, the shock wave expands freely, As it grows it sweeps up material from the interstellar medium which slows its expansion. Expansion causes adiabatic cooling (or the ...

2

When low mass stars are very young, they are termed pre main sequence (PMS) stars. These PMS stars have larger radii than main sequence stars of the same mass, and energy transport in their interiors occurs primarily through convection. The convection ensures that the star is thoroughly mixed and chemically uniform. As the PMS star radiates away its ...

2

One property of distant objects, that follows from the limited speed of light, is that we are observing them in the past; the light that they emit takes time to reach us. For very distant objects, we are effectively observing the universe at a very early time. Objects that are very far away, such as distant galaxies, are receding away from us, which causes ...

2

The aspect of the universe described as "flat" is the spacetime curvature. In General Relativity gravity isn't treated as a force but as a curvature of the spacetime manifold. At large scales the universe is expanding: negative curvature like the top of a hill, where things roll downhill away from each other. But mass causes a positive curvature like a bowl, ...

2

There is evidence that this assumption is incorrect. Stars actually started forming before 500 million years post-Big Bang. There is some evidence that Population III stars formed as early as 200 million years after the Big Bang. The oldest known star, the Methuselah Star, dates back to 200 million years after the Big Bang. It is a second generation star ...

2

Even if you substitute the definition of proper time by "distance traveled", you cannot guarantee you got that distance right. Suppose photon was absorbed by hydrogen interstellar gas, then re-emitted with different wavelength, was it a new photon or an old one? Time has no change for system associated with photon. So technically speaking, the photons you ...

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