# Tag Info

43

Energetic neutrinos have been observed from the core of a supernova (SN 1987A). The inferred temperature at the "neutrinosphere" is about 4 MeV (equivalent to 50 billion K - ($5\times 10^{10}$ K, Valentim et al. 2017). Hence it is observable and has been observed. The very centre of the proto-neutron star that is responsible for the neutrino emission is ...

24

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 absolutely no effect on the need for dark matter. The main results are: (i) as you look back in time, the overall (co-moving) density of galaxies (more massive than a ...

22

Note that while we haven't observed anything even close, there is a theorized Absolute Hot along the lines of absolute zero. Its theorized value is ~ $1.416 \cdot 10^{32}$ Kelvin. Above this temperature, it would be impossible to pump more energy into a system, even gravitationally. That gives an upper bound on the maximum temperature we could measure.

21

Until the Universe was 380,000 years old, it was filled with a gas of protons an electrons. There was also radiation, in thermal equilibrium with the matter, and because it was so hot, the protons and electrons couldn't form neutral hydrogen, since every time it "tried", an energetic photon would knock off the electron. This gas was everywhere. And photons ...

20

Surely if you stared long enough, the light from them would eventually hit your eye? Collecting light over a long span of time is how telescopes can see very dim objects. The human visual system doesn't work that way. For one thing, even when you think you are staring at something, your eyes still dance around a bit. It's a built-in response called ocular ...

17

No. In fact the opposite is the case. (See the last paragraph for an intuitive explanation.) It is a common misbelief that galaxies receding faster than the speed of light are not visible to us. This is not the case; we easily see galaxies moving at superluminal velocities. This does not — as I think most people would think — contradict the theory of ...

14

Not at all a dumb question, but actually you can see distant galaxies with the naked eye. From the northern hemisphere, the Andromeda Galaxy, our biggest neighboring galaxy, is visible if you know where to look, and is at a reasonably dark place. From the southern hemisphere, the two smaller, but nearer, irregular galaxies called the Small and Large ...

11

No. The furthest we can see is the cosmic microwave background radiation (CMB). Early on (after the big bang), matter was fully ionised and the electrons frequently interacted with the photons. That has two consequences. First, the radition was that of a blackbody at the same temperature as the matter. Second, the universe was opaque, i.e. photons couldn't ...

10

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

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 ...

9

Sort of... There is a system called the International Celestial Reference System (ICRS) which has center at the Solar System Barycenter (normally inside the Sun but not the same as the Sun's center) and which has the x and y axis in the plane of the Earth's equator and the z-axis ("North" if you will) pointing towards the Celestial Pole. The x-axis points ...

9

In the currently dominant theories, the Universe is basically the same everywhere, if you look on a large enough scale. There may not be a furthest star from Earth at all (the universe may be infinite) or it may be like "the furthest point on Earth from London" which exists, but is not a specially interesting place. The distinction between those two ...

8

Yes, indeed! Many nebulae are visible from Earth in a small and cheap telescope, and even to the naked eye (if you are standing in a sufficiently dark place). In fact, yesterday I was watching the Orion Nebula with my 4.5" telescope (which is worth $200 or so) from my apartment in the middle of Copenhagen. The term "nebula" is a bit of a… well, nebulous ... 8 Your 3 points are spot on. There is a finite number of galaxies we could theoretically reach for the reasons you say. The further away the distant galaxy the greater the expansion of space between us and that galaxy and beyond a certain distance, galaxies can't be reached, even by the speed of light. We can see galaxies that we couldn't possibly travel ... 7 Jonathan's answer is essentially correct, but as Rob Jeffries comments, he doesn't take into account that the Universe is expanding during the journey. The edge of the observable Universe is 47 billion lightyears (Gly) away. Even if you are a lightbeam, you cannot reach that point. The farthest you can go if departing today is roughly 5 Gpc, or 17 Gly, but ... 7 If I understand you correctly, you want to know the distance from the point from which we observe the CMB, to the edge of the observable Universe. During inflation, the observable Universe expanded from ridiculously small to some ten meters in radius, so that part can be safely ignored compared to the distances now$^1$. The distance$^2$to the "CMB shell"$^...

7

Yes, they would experience gravitational attraction. It would take a long time for them to collide... the formula (derived here and shown here) is: $$t = \frac{\pi}{2} \sqrt{\frac{d^3}{2G(m_1+m_2)}}$$ where $d$ is the initial distance between the two planets of mass $m_1$ and $m_2$ and $G$ is the Gravitational constant. This gives a time of about $10^{23}$ ...

6

Good question! I will need to go into some cosmology to answer it, my main source being Davis & Lineweaver (2003). Nothing traveling through space should be able to leave the observable universe. However, information can and will inevitably leave our observable universe. How is this possible? The Universe is expanding. If we lived in a static universe,...

6

There are around 2 trillion galaxies in the currently observable universe according to the latest estimates, obtained by integrating theoretical galaxy stellar mass functions above $10^{6} M_{\odot}$ between $0 \leqslant z \leqslant 8$. It's difficult to get a precise number for the total observed galaxies as the results from new surveys are being released ...

6

The observable universe's edge, by definition, is the farthest part of the Universe that light could have reached us from. Let's say light from the observable universe's edge has just reached us. The longest it could have possibly travelled for would be 13.8 billion years, since the Universe is only 13.8 billion years old. As such, one might assume that the ...

6

How fast does the observable Universe grow? The observable Universe is $r = 46.3\,\mathrm{Glyr}$ (billion light-years) in radius, so by Hubble's law, galaxies at that distance recede from us at a speed $$v_\mathrm{rec} = H_0\,r = 9.6\times10^5\,\mathrm{km}\,\mathrm{s}^{-1},$$ or $3.2$ times the speed of light, $c$. At the same time, light from ever-...

5

As time passes, there are galaxies that are currently not in the observable universe which will become observable But this is not a sudden winking on. Instead, over hundreds of millions of years we will see a proto galaxy evolve into a mature galaxy. For example there is a "blob" of hydrogen that some interpret as being the accretion of hydrogen onto a ...

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

It was a loud explosion First, we have to clarify what the Big Bang was. The name is a misnomer, as it was neither loud nor an explosion. All we know is that the Universe is currently expanding — that is, space is literally being created between all matter. We have plenty of answers here explaining this more intuitively, such as mine (which also answers ...

5

A finite universe is said to have a "closed geometry", or to be "positively curved", meaning that, in principle, you may travel in a straight line and eventually return back to your starting point. In the 2D analogy, the surface of Earth is positively curved, and if you travel 40,000 km straight, you're back where you started. A finite universe that does ...

5

I see 2 candidates: the universe is 13.8 billion years old, so should be rounded off to 14 the cosmic horizon is 46 billion ly away

5

The furthest we can "see" is the cosmic microwave background at a redshift of about 1100. The proper distance of the CMB-emitting gas that we see today is about 46 billion light years. If you are talking about galaxies, then the first are thought to have formed at redshifts of about 20 (current distance 36 billion light years) and beyond that are the ...

4

Just because you can keep your eyelids open for $x$ seconds doesn't mean you are collecting light for $x$ seconds and using it to form a single image in your brain. How would you "save" the photo? How would you decide when to end light collection? You know as well as I do that you can't simply lift your finger off your brain's shutter release! And that's on ...

4

Few photons -- You have tiny pupils. Only photons that manage to travel that far over that much distance along a path that manages to intersect with your tiny pupils will have a chance of being seen. And only some photons that reach your retina actually interact with molecules that register their arrival. Interference -- The molecules of the atmosphere, dust ...

4

Your reasoning would be valid not only for galaxies, but also for stars and anything it shines in the Universe, but there is an important effect which invalidates it: absorption of light. Intergalactic and interstellar medium is filled with dust and gas, which contributes to absorb and scatter the light from distant objects. Especially on the plane of our ...

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