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2

From a philosophical standpoint, it is impossible for me not to exist. I'm not sure about you, but I know that I am a thinking, cognizant mind perceiving some sort of reality y having an experience. Whether that experience corresponds to ultimate reality is irrelevant. Since we've yet to solve the problem of hard solipsism (with no known way of even how to ...


8

There are a number of different frames of references. For the velocities of celestial objects we use: (i) The geocentric frame: This is a velocity measured with respect to the Earth's centre. Obviously this is quite useful for artificial satellites, but also for things like meteors. (ii) The heliocentric frame: this is the velocity as seen from the centre ...


26

There are two separate questions there. The easiest one to answer is how we measure the vleocity of the Earth, Milky Way etc, because we measure it relative to the cosmic microwave background (or CMB). If you measure the CMB in all directions and find it's the same in all directions then you are stationary in comoving coordinates. However if you find the ...


2

Speed is a distance (separation between two well defined points in space) traveled over a time. The speed of Earth you quote is the orbital velocity. We know how far away the Sun is and we know the shape of the orbit, so we know how far the Earth travels relative to the Sun (distance) per year (time). Likewise the speed of the Milky Way is also given ...


0

$\nabla T=0 $ is not a conservation law. You are not considering the energy of the gravitational field in this way. If you do the calculation you find something like $\partial_{\nu} (\sqrt{-g} T^{\mu \nu}) \neq 0 $, so you can't define a conserved charge like $P^{\mu}= \int \sqrt{-g} d^4x T^{\mu0} $. To account for the gravitational scalar field you ...


10

The most compelling evidence of GR in presence of matter is, in my opinion, in neutron stars. These objects have a surface gravity $SG$ that is (geometric units): $SG_{NS}=GM/c^2 R \simeq 0.1 $ This value is telling us that we can't use Newtonian gravity because we are in the strong field limit. For comparison, the sun has $SG_{SUN}=GM/c^2 R \simeq 10^{-5} ...


0

The field equations of general relativity give rise to a phenomenon called gravitomagnetism, which is related to "monopole" gravity in the same way that magnetism is related to moving electric charges by special relativity. There is conclusive evidence for gravitomagnetism just in the past five years, weakly from the Gravity Probe B mission, and more ...


6

Frame-dragging effects are dependent on the spin of the central object, have been measured by experiments such as Gravity Probe B, and are definitely not dependent on the central metric. Also, any effects on a galactic scale are best quantified in terms of a continuous matter distribution, since the central black hole is a small fraction of the galaxy's ...


4

Energy conservation stems from Noether's theorem applied to time (i.e., time-invariance leads to energy conservation, similarly to how spatial-invariance leads to momentum conservation). Since the universe is expanding (and accelerating at that), the state of the universe today is different than it was yesterday and will be tomorrow, hence energy ...


1

What we like to call the energy, i.e., the total matter/energy content of space-time, might not be conserved. However, there is a lot of reason to suspect that fundamentally the universe is some big quantum system, and that space-time and particles and fields are emergent from this underlying idea. In that case, we expect there to be a Hamiltonian $H$ and ...


2

A recent paper by Dan Whalen titled, Finding the First Cosmic Explosions. I. Pair-instability Supernovae discusses this very problem. The pair-instability supernova (PISNe) is a special case of massive stars, around 100 $M_\odot$, in which the thermal pressure inside the star is reduced via the production of electron-positron pairs. Runaway thermonuclear ...


0

When cosmologists say that "the universe is flat" they actually mean the "observable universe." This doesn't imply that the universe as a whole is flat, just that the observable universe is very small in relation to any curvature.


0

Can you name the source that gave a finite estimate for the mass/energy content of the "universe" so we can look at the context? I would assume they were probably talking about the observable universe, defined to mean only the points in space close enough so that a light wave (or gravitational wave) emitted from that point at any moment after the Big Bang ...


0

A good, semi-technical discussion of the general problem (how the post-Big-Bang evolution of the universe, including the formation of galaxies, stars, etc., can be reconciled with the 2nd Law) can be found here: http://arxiv.org/abs/0907.0659 It's important to realize that while the ensemble of atoms in the gas cloud does indeed, as your intuition suggest, ...


1

You don't need oxygen to make an explosion. All that is needed is that the reagents after the explosion occupy more space than those before. Going from solid to gas is a good way of getting this effect. Similarly, going from gas to solid is a good way of producing an implosion, but there is a limit on how much bang you can get from this.


1

It is metaphysics, instead of God creating the universe , it has Nature creating the universe, and hand waves mathematical possibilities to justify this metaphysical premise. Some inconsistency in the end when using Newtons gravitational equation and assuming the masses zero, he says that the distance is zero too, but the distance between two nonexistent ...


3

In this paper by Gott et al., on p. 466 they define the "future visibility limit", saying in the published version that "No matter how long we wait, we will not be able to see farther than this", and on p. 7 of the arxiv preprint they similarly say "If we wait until the infinite future we will eventually be able to see the Big Bang at the co-moving future ...


9

If our ideas about cosmic evolution are correct, galaxies that are visible today will in principle remain visible in the future. As time goes on, light from more and more distant galaxies will be able to reach us, and the number of observable galaxies will increase. However, there exists a cosmic event horizon$^1$, so this is an asymptotic growth: There's a ...


2

In 1988, Weinberg used an equivalent relation as an historical starting point for the weak anthropic principle, in his "The cosmological constant problem". I quote, page 7: An example is provided by what I think is the first use of anthropic arguments in modern physics, by Dicke (1961), in response to a problem posed by Dirac (1937). In effect, Dirac ...


-1

There is a very simple answer to your question, OP. Big Bang was not only the physical beginning of our universe (the stars, planets, gas clouds, black holes etc) but also the temporal beginning of our universe. That is, there was no "time" before big bang. Time, space and all matter/energy started with the big bang. It might appear funny to think that time ...


0

If the Universe is infinite, then it had instantly grew from zero to infinity at the moment of Big Bang. Note, that this is a possible model from General Relativity (GR). But most probably GR is not applicable at the moment of Big Bang, because it contains singularity.


0

The accelerated expansion of the Universe is an effect of repulsing forces BY DEFINITION. The nickname of the reason of these forces is "dark energy". The problem is that Einstein's $\Lambda$ term is very simple addition to equations. It could be added even if Einstein didn't want to make Universe static. So it is a great possibility, that Einstein didn't ...


2

The simplified model of the universe used in discussions like this is the FLRW metric. From the tone of your question I'm guessing that you're not looking for a detailed mathemtical treatment, so let me just pull out the equation that answers your question. This is one of the two Friedmann equations: $$ \frac{\ddot{a}}{a} = -A\rho + B\Lambda \tag{1} $$ ...


0

Brian Greene talks of many more types of multiverses...holographic, extra dimensional, simulated etc. It is also possible our universe is only a construct of our consciousness in which case consciousness would vary in a different universe..but that is in the realm of philosophy. Higgs field variation- a basic energy field value creating the constants and ...


0

Have a look at the Wikipedia article on Binary Black Holes. Essentially when two black holes come in close proximity they are believed to merge into a bigger black hole so you won't get one black hole sitting at the event horizon of another for infinity. Super massive black hole binaries are believed to form during galaxy mergers Regarding the answer ...


3

If we define a scale factor $a$ for the universe (could be the distance between two galaxies), then this scale factor will change with time. This is also true in flat or infinite universes, so long as the Hubble parameter is $>0$ (i.e. the universe is expanding). The energy density contained in the cosmic microwave background will scale as the energy of ...


0

The universe was infinite at its creation, and continued to be so. It just got bigger during inflation. There may be a misconception that the universe begun as a "infinitesimal point", but singularity just means that it was non-defined. During inflation, the space between matter became wider, analogous in 2-dimensions, to the "spots on a balloon's surface" ...


0

So, we first have to understand what it actually means to be 'expanding.' The FLRW Metric describes our universe as homogenous (physics works the same at all points), isotropic (looks basically the same everywhere on a large scale), and expanding. In simplest terms, the further back in time we look, the closer things generally seem. Basically, objects that ...


0

It is a pseudoproblem of definition. Some people define universe as everything that could ever possible exist, to them the word multiverse is an oxymoron. But those who like to use the idea of a multiverse, use it encompassing different things depending on context. For instance, Max tegmark defines 4 different levels of universes/multiverses: Level I: ...


3

As far as I know, "a" Universe is caracterised by fundemental constants such as the speed of light, Newton's gravitationnal constant, the Planck constant and so on. You could distinguish between two different universe from the variation of these values I think.


0

I think there is no more "slow" light from the begining od the universe remaining because photon has accelerated after expansion to reach their current speed. Maybe I'm wrong but to me it seems to be the right way to think.


4

Your idea that a quantum fluctuation created the universe is a misinterpretation of the suggestions that I have heard. Explaining why requires introducing a few ideas, so bear with me while I do this. The object we think of as the universe is made up of two bits: a manifold equipped with a metric = spacetime some matter/energy The manifold and metric ...


0

I think you misinterpreted the quote. Here it is in full: To explain, for example, how the universe could have smoothed itself out to achieve the uniformity of temperature that we observe today in the cosmic background radiation, one finds that in the context of the standard Big Bang theory, it would be necessary for energy and information to be ...


0

The speed of light is constant relative to the fabric of space, but when space itself is expanding, the speed of lighg, as measures by an external obsrver can be larger than c. This is a known fact in general relativity (special relativity doesnt consider such possibility). Update: what has slowed down was the expansion of space itself, with drags the light ...


3

Special Relativity states that as an object increases in velocity, it gets more dense (mass increases, volume shrinks). No, no, no. Absolutely not. But I know why you think this. Back many decades ago, before I was born, lots of physicists wrote physics papers and textbooks without really understanding relativity. They made mistakes. Not necessarily ...


-2

Black holes do explode after their life is over, thus vomiting out all the matter they ingested. Apologies for not providing credible explanation for my claim (in the form of mathematical equations). I read that (that black holes do explode) in Stephen Hawking's book titled Black Holes And Baby Universes. He stated that there is an inverse relation between ...


4

No, throwing matter out isn't contrary to a BH; there is often an accretion disc surrounding the black hole and that is what forms the jet. No, the ejected material cannot condense to form a galaxy. A galaxy requires the material to be gravitationally bound to some central point, a jet moving at $\sim c$ is moving too fast to be gravitationally bound to ...


1

As far as I know, the number of points to not have any influence on divergent behaviour. The infinite vacuum energy comes from the fact that we allow arbitrary frequencies for our quantum fields. There is no difference if we sum or integrate a constant from zero to infinity, the result is still infinite. $$\sum_{k=0}^\infty {1\over 2} \sqrt{k^2+m^2} ...


12

I assume your question was asked with the implicit "and everything else is kept the same" (still GR + standard model, just with one parameter tweaked). This would have a large effect, because now the neutron would be much more stable! The neutron is already quite stable (~ 10 minute half life), due to the tight energy constraints in the reaction decaying ...


4

Javier, brings up some interesting points. However, protons and neutrons get most their mass from relativistic quarks. If the quarks could be slowed down they would weigh a few electron masses. So what ever is responsible for giving the electrons mass its value seems to be giving the quarks their rest mass. I just checked the up and down,they are around 4 ...


3

There are scientists that do not accept the Big Bang (and more). That doesn't mean that there isn't evidence for the Big Bang (there is), nor does it imply it may be wrong. All we can say is that some scientists consider the Big Bang to be a fact because the evidence is overwhelming, some who think it is the best cosmological model because there is much ...


2

@GeorgeSmyridis: the big bang is a model that describes a bunch of cosmology. Physicists, when referring to it, rarely are talking about the event that happened at the very beginning of the model. In fact, the situation in the model goes outside of the applicable region of known physics before you get to the point in time when the explosion would have ...


4

Correction of typos and clarifications In the Friedmann equation, '$\rho$' is strictly speaking $\rho_m$, the mass density. Hence the presented Friedman equation has to be changed as follows: \begin{equation} H = \sqrt{\frac{8\pi}{3}G\rho_m}=\sqrt{\frac{8\pi}{3}\frac{G\rho}{c^2}}, \end{equation} in order to use the expression given below for the energy ...


1

The Universe is infinite. If the energy density of the universe is greater than zero, then the total energy must also be infinite.


2

Indeed spherical harmonics are inappropriate, since they are not orthogonal on the restricted domain. This is particularly noticeable in small-scale surveys like ACT and BOOMERanG, but even "full-sky" surveys mask bad data. COBE for instance masked out the entire galactic plane, so the problem has been known since then. The solution presented by Górsky ...


0

Entanglement results when the system under consideration belongs to one quantum mechanical solution of the specific problem. The inflationary period as introduced at the beginning of the Big Bang model is a quantum mechanical solution to the boundary problem of the very early universe. At those sizes and energy densities it is assumed that everything is ...


1

You need to include the 'zero' versions of $\rho$ and $a$ in order for the units to works out, or, to put it another way, to account for initial conditions. As regards the 32, you made an error while integrating: $\int a \mathrm{d}a = \frac{a^{2}}{2}$; the factor of 2 taken to the other side under the square root provides a 4 to makes the 8 into a 32.


2

The singularity comes from the scale factor $a(t)$: $$ds^2 = -dt^2 + [a(t)]^2 ( dr^2 + r^2 d \Omega^2)$$ By solving the Friedmann equations for the scale factor we know that: $$a(t) = a_0 t^{\lambda}$$ where $\lambda$ is some positive number that depends on the matter-radiation ratio of the universe. At $t=0$ the scale factor becomes $a(0)=0$. So at ...


0

I understand that there was zero entropy or at least low entropy that became self agitated to bring about the big bang. The closest example to zero or very low entropy is a solution of Dropleons with quasiparicles structures in a liquid state that are self propagating. This is based on the initial conditions of the solution containing super conducting and ...


0

Convert the masses into energy by multiplying them by c^2. Learn what percentage of each one there were, know the energy of the universe at that point... Find what percent of that is each particle... Remember, positrons and electrons are antiparticles and therefore have the same (rest)-mass. As for Photons at that time, determine their energy. Then sum it ...



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