According to the $\Lambda$CDM parametrization of the Standard Model of Big Bang cosmology, the universe contains a cosmological constant $\Lambda$ associated with $73\%$ dark energy, $23\%$cold dark matter (CDM) and $5\%$ ordinary matter.   I have also read from Kolb and Turner's book that the Cold dark matter leads to a "bottom-up" formation of structure in the universe while hot dark matter would result in a "top-down" formation scenario; since the late 1990s, the latter has been ruled out by observations of high-redshift galaxies such as the Hubble Ultra-Deep Field.  $\bullet$ How do we know that the dark matter is cold or nonrelativistic?  $\bullet$ What is the percentage of hot dark matter in $\Lambda$CDM parameterization? Is the possibility of dark matter totally ruled out?    Addendum Existence of CDM does not rule out existence of HDM. In fact, it is assumed that the dominant fraction of the dark matter is CDM type. But observations only tell us that $23\%$ of the total mass-energy of the observable Universe dark matter. But this information does not tell how much of it is HDM and how much is CDM? The belief is that the the dominant component of the dark matter is CDM but is there an estimate? Is there a physical way to understand/expect why the CDM should be the dominant contribution?  Please note There is a typo. I wanted to ask "Is the possibility of hot dark matter totally ruled out?"
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Most of the dark matter in the Universe is cold or nonrelativistic which rules out neutrinos as dark matter candidate as they are hot or relativistic. But why do physicists think that the dark matter is cold? How cold (i.e., how heavy) do these dark matter particles have to be?
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It is well known that there exists a reference frame where the total momentum of the Cosmic Microwave Background is zero (a basic fact of special relativity applied to a collection of massless particles) and the radiation is homogeneous and isotropic. This can be observed in the dipole anisotropy.  Neutrinos also decoupled from matter shortly after the Big Bang, and therefore relic neutrinos must form a Cosmic Neutrino Background ($\mathrm{C}\nu\mathrm{B}$). These neutrinos are extremely hard, if not impossible, to detect with current technology.   The $\mathrm{C}\nu\mathrm{B}$ also defines a reference frame of zero total momentum. Is it expected that this reference frame is coincident with that of CMB?
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I understand what is meant when we say , but I've seen it claimed it's is fine tuned "for life". Is it?  When I look at known or even speculated life in our universe, I see life clinging to a tiny crumbs separated by vast voids. Our life needs baryonic matter, only 5% of the universe. Its density is $10^{-30}  g/cm^3$, but it clumps together leaving vast voids between the clumps. 99% is in stars and black holes and nebula utterly inhospitable to known life.  That leaves the parts "fine tuned for life" to be 0.05% of the universe. The planets, asteroids, and comets seem like just debris separated by  than the life-sustaining clumps preventing life from spreading.  Even that seems to be used very inefficiently. Life requires the energy of a star, but it's spewed out in all directions. We only receive a tiny fraction of it. Similarly, life requires heavier elements created in supernova, but these are also spewed out in all directions. And it took billions of years to develop a being capable of asking "is the universe fine tuned for life?"  It seems like we live on the crumbs of creation warmed by a rounding error of the heat from our star. Taking this view, the universe appears extremely inhospitable to life. It could be a lot better tuned.  Is it that the universe is fine-tuned for life? Or is it that the universe is so large and life requires such an infinitesimal fraction of the universe that it's a near guarantee were we to roll the dice again that some crumb, maybe not even baryonic matter, could develop something complex enough to ask "why is the universe fine tuned for life"? Am I just restating the weak anthropic principle?  I understand this is straying into philosophy. If that's off topic, are my  physical assumptions correct?
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I've been looking for questions about dark matter, and I've read some very interesting answers. However, I desire too look into it deeply.   This is not actually a question. I'm asking the community to recommend interesting references to understanding dark matter and dark energy.   I accept all sort of references: notes, books, scientific papers etc.  Let us assume some background on classical physics, thermodynamics and basics about quantum theory.
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This may be a silly question, but is it possible that dark energy and gravity are related to each other?   Space-time is deformed everywhere in space by objects with mass. The more massive the object, the more deformation in the local area, and, as a result, the more concentrated space-time is within that deformation. However, where is the "extra" space-time coming from to be able to form these super-concentrated regions of space-time? Is that what we observe as dark energy?  Is it possible that at great enough distances, gravity could be pulling space-time apart due to the lack of mass in the great voids of space? Not only that, but if there was enough mass in a part of space to have a little more gravity than the surrounding area, it could prevent that area of space from being "stretched out." Thus, might dark matter just be regions of space where space-time is denser than the surrounding regions?  I'm sure these aren't new ideas, but I'd like to know what premises and assumptions are incorrect.
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Is there evidence of dark matter in our galaxy?  How can we measure this, say, how many percent of the center of our galaxy is dark matter?  I did not find the answer in the question
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Why is absolute zero considered to be ? Wouldn't regions such as massive gaps between galaxy clusters have temperatures of absolute zero?  I just do not see why our model must work the way that it does. I mean there have to be regions with no thermal energy out there, the universe is massive.
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It is well-known that measurements of the velocity profile in galaxies are not compatible with Newtonian laws. A way to circumvent the problem is to assume that galaxies are surrounded by a spherical halo of Dark Matter. The density of mass of this Dark Matter can then be computed from the velocity profile with Newton's laws.   My question is the following: the formation of a galaxy involves essentially only the gravitational interaction. According to Newton (and General Relativity too), the trajectory of a particle does not depend on its mass. As a consequence, I do not understand how usual matter and Dark Matter could have followed different trajectories leading to such different mass distributions nowadays. Can somebody explain this (apparent) paradox ?
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-I'm not a religious person so this is not a denial. I'm just trying to understand the most fundamental topic about Universe.  -I know the Big Bang cosmological model is not a law but it's a theory.  -I'm a non-Native English speaker so please let me know if the question is not clear for you.  -This question came to my mind after watching
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I can think of three general ways of explaining why the universe contains more matter than antimatter:  (1) Near the Planck time, the universe had zero baryon asymmetry, but at some later time, determined by some GUT energy scale, the  were satisfied, and the baryon asymmetry became nonzero.  (2) Nonconservation of baryon number occurs only at Planckian energies. Near the Planck time, the baryon asymmetry evolved from zero to some nonzero value.  (3) The Sakharov conditions have never been satisfied. The baryon asymmetry has always been nonzero, and has simply scaled as expected. (Apparently one  $\Delta n=n_B-n_\bar{B}\propto s$, where $n$ is number density and $s$ is entropy density).  It seems like most theorists are interested in #1, but is there any reason why 2 and 3 aren't possible?  2 seems pretty reasonable, since for the reasons given in , we have good reasons to think that baryon number is not conserved under Planckian conditions.  3 also seems reasonable to me, since even if baryon number is nonconserved at Planckian energies, that's only one of the three Sakharov conditions. I don't see any obvious fine-tuning objections to #3, since the scaling of baryon asymmetry with cosmological expansion isn't particularly drastic (not an exponential decay or anything). Is there something unphysical about maintaining $\Delta n\propto s$ all the way back to the Planckian era?   Some people might object to #3 on aesthetic grounds, since we "expect" the initial conditions of the universe to be symmetric, but that seems weak to me. After all, we don't object aesthetically to the fact that homogeneity was an imperfect symmetry of the early universe, and we even accept that the early universe was in a thermodynamically unlikely state.
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Maybe my understanding is just off, but the  is just a scalar, right? What are it's units? Why a scalar? - was a tensor 'cosmological constant' ever considered or is it just not conformable to the math or physics of the field equations?
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Ok, from astronomical observations we can tell that the observable matter is separating - so rewind the clock about 13.7 billion years and it was all at a single point.  However, how do we distinguish between the following two options:     Universe is expanding   Matter distribution is increasing into infinite void   Clarification 1: (My notion of) The traditional notion is that all time/space/matter was created at the instant of the big bang.  I.e. BB was inital conditions of: $t=0$, $V=0$, $E=very big$  People say "the universe is expanding", rather than "observable matter is separating".  Why is this?  How do we know that the big bang event wasn't started by all matter condensed at a single point within a larger (otherwise empty) universe?  How do we know that BB wasn't: $t=0$, $V(universe)&gt;0$ but $V(matter)=0$, $E=very big$?
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What is the relationship between redshift and conformal time ?   For example in a paper i found:   taking $z_e = 3234 $ at the time of radiaton-matter equality yields the conformal time $\frac{\eta_e}{\eta_0} = 0.007$ and taking $z_E=0.39$ at matter-$\Lambda$ equality yields $\frac{\eta_E}{\eta_0}=0.894$ and setting redshift at decoupling $z_d=1089$ yields $\frac{\eta_d}{\eta_0} = 0.0195$  where $\eta_0$ is the present decoupling time.  Further some cosmological parameters are given as :  $\Omega_r = 8.36 \times 10^{-5}, \Omega_m = \Omega_b + \Omega_{dm} = 0.044 +0.226, \Omega_\Lambda = 0.73,  H_0=0.72$  Now how can i calculate all those $\eta_e, \eta_E, \eta_d, \eta_0$ from given redshift values and/or above parameters ? I searched whole of my text books trying to find an explicit relation for conformal time $\eta$ but all i got was $ \mathrm{d}t=a(t)\mathrm{d}\eta$. Any help would be very helpful.
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I'm a layman, but i watched some intereting videos about big bang on youtube[michio kaku, hawking this kind of things, not some crackpots :)]   I described everything on my picture:     So is there any possibility to see that inner side, for example, by creating wormhole, or entering new dimension? Or i get something wrong, on the way, and my model is totally wrong?  And another question. I heard that even if you would fly a ship across the universe you would eventually end in the place where you started, because we live in a 3d space that is defined on a surface of big sphere. But why a big sphere? Universe might be a big mobius strip, and the same principle of ending at start will apply. Is it possible?  Thanks in advance, and please don't bash me for silly questions. Im just curious, but i don't have any degree in physics. Im also not an english native speaker.
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Astrophysicists believe that dark energy exists and makes up ~70% of the universe's energy content. What is the evidence that dark energy exists? Wikipedia gives :   Supernovae. A specific type of supernovae (type 1A) are supposed to be standard candles - that is, their luminosity is known. In turn this lets us measure distance to faraway galaxies, check how fast those galaxies are receding from us, and check if that recession speed is increasing over time. CMB. Data from the Cosmic Microwave Background indicates the universe is approximately flat. Visible matter + dark matter can account for ~30% of the energy content required to make the universe flat, leaving ~70% for something else - dark energy. BAO. Baryon acoustic oscillations act as a "standard ruler" that lets us measure how the Hubble constant varies with redshift (i.e. time), and see if the recession speed is increasing with time. Late-time ISW. ISW stands for integrated Sachs-Wolfe effect. The idea is that, usually, a photon that enters a potential well gains energy as it falls in and loses energy as it emerges, and gain/loss exactly cancel. If a universe is expanding in accelerated fashion, then this is untrue; the potential wells / hills are smoothed out and there is a permanent shift in the photon's energy and therefore temperature. If we see a correlation between hot and cold spots on the CMB and the locations of superclusters and voids, then it's a sign of accelerated expansion. Galaxy evolution. This uses (known) evolution of early-type galaxies as a standard clock. Once we know how long it takes for a galaxy to evolve from one state to another, as well as their redshifts, we can reconstruct how the Hubble constant varies over time, and see if the recession speed is increasing with time.   Two questions:   Did I understand any of these five methods wrong? Are there any other lines of evidence for the existence of dark energy?
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For example, the radiation dominated cosmology, the energy density of radiation is proportional to $a^{-4}$ and the volume is proportional to $a^3$, where $a$ is the scale factor. So the total energy of radiation is propotional to $a^{-1}$. So where is the loss of energy of radiation? Is it because the gravitational field has the energy? Does $\nabla_aT^{ab}_{\rm matter}=0$ represent the conservation of energy and momentum of matter field in GR?
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Does the Friedmann vacuum equation have a linear solution rather than an exponential one?  Using natural units one can write Friedmann's equation for the vacuum as $$ \begin{eqnarray} \left(\frac{\dot a}{a}\right)^2 &amp;=&amp; \frac{8\pi G}{3}\rho_{vac}\\\tag{1} &amp;=&amp; L^2 \left(\frac{\rho_0}{L^4}\right) \end{eqnarray} $$  where I define the Planck length $L=(8\pi G \hbar / 3 c^3)^{1/2}$, $\hbar = c = 1$, and $\rho_0$ is a dimenionless constant.  Now let us interpret the Planck length $L$ to be the size of the smallest volume of space that can be described by general relativity.  But the Weyl postulate, together with cosmological observations, also imply that space is expanding.  Therefore we must have  $$L = a(t) L_0\tag{2}$$  where $L_0$ is the Planck length measured at the reference time $t_0$ where $a(t_0)=1$.  Inserting Eq.(2) into Eq.(1) we find  $$\left(\frac{\dot a}{a}\right)^2 = L_0^2 \left(\frac{\rho_0}{a^2L_0^4}\right)\tag{3}$$  where the Friedmann equation (3) has been rescaled in terms of the Planck length $L_0$ measured at the reference time $t_0$.  Eq.(3) has a linear solution  $$a(t) = \frac{t}{t_0}.$$  The scaled mass density $\rho(t)$ of the vacuum is not constant but rather given by  $$\rho(t) = \frac{\rho_0}{a^2 L_0^4} = \frac{1}{t^2 L_0^2}.$$