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To ask what do physicists expect to accomplish with gravitational lensing is nowadays somewhat like asking what do biologists expect to accomplish with looking at things with microscopes. Gravitational lensing is a well established method used across astronomy and the main challenges the field itself has to tackle are mainly technicalities. But I will try ...


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If you "run the clock backwards" using the Friedmann equations, you can avoid a singularity; you just need the pressure and energy to behave in the right way when the Universe was very hot and dense. The catch is that "the right way" in this case means that $\rho + 3 p < 0$; in other words, either the energy density or the pressure (or both) have to be ...


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You use the term frame of reference but we need to be careful what we mean by this. In special relativity this phrase generally means an inertial frame i.e. a frame in which Newton's first law applies. In GR we obviously can't have a global inertial frame because objects accelerate (due to gravity) whenever they are near a mass so their behaviour isn't ...


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Slow-roll inflation, which is one of the more popular models, has a well-defined mechanism for determining the end of inflation. Under this framework, we define two slow roll parameters, $\epsilon$ and $\eta$. Inflation is defined to end when $\epsilon=-\frac{\dot H}{H^2}\sim1$ Here, $H$ is the Hubble rate and $\dot H$ is its time derivative. Some simple ...


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When MTW say the universe is isotropic, they mean it is isotropic everywhere i.e. at all points in the universe. It's easy to construct universes that are isotropic at a single point and not homogeneous, for example CuriousOne's suggestion of a ball with density that is a function of distance from the centre. However this ball is only isotropic if you are ...


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Mead and Ringermacher's paper is available on the Arxiv here. Their previous publication that descibes how they perform their analysis is also on the Arxiv here. Mead and Ringermacher's paper is phenomenological, i.e. it reports an analysis of experimental data (measurements of SN1a red shifts) and the paper suggests a scalar field that could be responsible ...


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If we take the Milky Way as an example, the black hole at the centre, Sagittarius A$^*$, has a mass of about 4 million times the Sun. However the mass of the Milky Way is somewhere around a trillion Suns. So the central black hole makes up 0.0004% of the total mass. So even if our central black hole was pure anti-matter it wouldn't come close to accounting ...


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By inspecting the equation, you should be able to deduce that $R_0$ has the same dimensions as $R$ (since you are adding $1$ to $\frac{R^2}{R_0^2}$) $\lambda$ has no dimensions (since you are adding $R$ to $\lambda R_0$ times some dimensionless term)


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Is it possible that universe is not expanding but instead being dragged into singularity? Yes. That possibility is called the Big Rip. 'dragged into a singularity' can happen even while 'expanding', so they are not contradictory. It would occur if the rate of cosmic acceleration is exponential. In the Big Rip scenario, there is a finite time in the future ...


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Is it possible that universe is not expanding but instead being dragged into singularity? No. We see galactic redshift every where we look, the galaxies are moving apart like the raisin-cake analogy. There is no overall gravitational field in the universe. You may have heard about "the big crunch" but I'm afraid it's popscience. The universe didn't contract ...


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There are three types of matter/energy we consider when calculating how the universe expands: Matter - both normal matter and dark matter Radiation Dark energy We measure the expansion of the universe using a scale factor that we normally denote by $a$. The scale factor increases with time as the universe expands, and if we look backwards in time we see ...


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The Big Bang was originally just the zero time limit of the FLRW metric. I'm not sure that Big Bang Theory has a meaning outside of CBS, but to the extent that it does have a meaning it is synonymous with the solution to Einstein's equations for a homogenous isotropic universe. Life is more complicated now because we believe the universe underwent a period ...


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People are addressing the speed question, but just to be clear: a photon can be very low energy. For instance, radio waves are much lower energy than gamma rays, even though both are made of photons (and, in vacuum, both travel at the speed $c$). What determines the energy of a photon is the frequency of the excitation (frequency of the corresponding light ...


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Although it has been said in other comments and answers, it bears repeating succinctly: photons (as far as any experiment can tell) are massless and therefore always move at the universal, invariant speed of light. There is NO non-relativistic description of the photon. Even the "classical" description of light - Maxwell's equations - can be interpreted as ...


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I will reply to this because the checked answer is not answering the question.The question is about photons, the answer is about light. It is as if the question were about atoms and the answer is about density of material. The question is asked about photons, the quantum mechanical framework is relevant to it. The checked answer is about light which is in ...


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Refractive Index is when light travels more slowly in a medium. Here is an example of light being slowed down to 38 miles per hour. The speed of a photon does not affect its energy. It has zero mass, therefore zero kinetic energy. The energy it has is due to its frequency (color), and nothing else. (However, it does have momentum!)


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Difference between in-out correlators and scattering matrix elements: At the root level, they are just fourier transforms of one another (connected through the free field operators, as you know already from LSZ connection). Difference between in-in and in-out: In the simplest possible way, what you calculate usually in QFT's are 'amplitudes' which ...


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You should not confuse the terms light years $[ly]$ to be a unit of time, it is actually a unit of distance, the distance light will travel in vacuum in one year. When we speak of the age of the universe we use years, in particular about $13.7$ billion years. Now do to the expansion of the universe the distance form which the light comes to our eyes or ...


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The scientific paper linked in the article you read is saying that it seems that there's a clustering of mass beyond the reach of current all-sky galaxy redshift surveys that is pulling the local volume more to one side. Such clusterings are possible in the framework of the standard $\Lambda$CDM cosmology, but a clustering large enough to explain the data is ...


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You are correct about the bias factor - because the dark matter distribution is not measured directly, but via tracers (galaxies), there may be some bias in the tracers. This is put into the analysis as an unknown bias parameter $b$ that needs to be fit. Now the second part of your question - why do we care about the dark matter? Briefly, it is because the ...


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The BAOs became imprinted in the background density of the Universe at the time of decoupling, which happened at the same time as recombination, i.e. when the linear scale of the Universe was a factor $z \sim 1100$ smaller. At this time, they had a characteristing wavelength given by the speed of sound in the plasma at that time. This happened to be ...


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In general relativity there might not be a general frame of reference that will look the way an inertial frame of reference looks in special relativity. And the fundamental deep down reason is that we didn't assume there had to be, thus it didn't have to happen. Whether a particular solution to Einstein's equation has one or not is up to experiment to ...


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According to what I have read, we have measured the universe to be flat More or less. I'm happy enough with the WMAP results that indicate that the universe is flat. To be blunt I never thought it could be anything other than flat. the shape of the universe is directly related to the mass-energy density. That's what they say. But IMHO two out of three ...


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Just some minor remarks: (i) An expanding universe filled with matter and a zero spatial curvature are not in contradiction. Please read up about the Friedmann equations and the Robertson-Walker metric. (Sorry, but I think that's the way to understand cosmology properly.) (ii) Flat geometries are not necessarily infinite (non-compact). Take for example a ...



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