Cosmic Holography Bin Wang(王 斌) Department of Physics, Fudan University
Entropy of our universe • 2.7K cosmic microwave background (CMB) • The universe started in a low-entropy state and has • not yet reached its maximal attainable entropy. • Questions: • Which is this maximal possible value of entropy? • Why has it not already been reached after so many billion • years of cosmic evolution?
Bekenstein Entropy Bound (BEB) • For Isolated Objects • Isolated physical system of energy and size(J.D. Bekenstein, PRD23(1981)287) • Charged system with energy , radius and charge (Bekenstein and Mayo, PRD61(2000)024022; S. Hod,PRD61(2000)024023; B. Linet, GRG31(1999)1609) • Rotating system (S. Hod, PRD61(2000)024012; B. Wang and E. Abdalla,PRD62(2000)044030) • Charged rotating system • (W. Qiu, B. Wang, R-K Su and E. Abdalla, PRD 64 (2001) 027503 ) +
“Entropy bounds for isolated system depend neither On background spacetime nor on spacetime dimensions.” Universal
Holographic Entropy Bound (HEB) Holographic Principle Entropy cannot exceed one unit per Planckian area of its boundary surface(Hooft, gr-qc/9310026; L. Susskind,J. Math. Phys. 36(1995)6337) AdS/CFT Correspondence “Real conceptual change in our thinking about Gravity.” (Witten, Science 285(1999)512)
Comparison of BEB and HEB Isolated System For For Cosmological Consideration Cosmological entropy HEB violated for large BEB too loose for large Two bounds cannot naively be used in cosmology. Both of them need revision in a cosmological context.
Problem: In a general cosmological setting,Natural Boundary? • Particle Horizon BEB (J.D. Bekenstein, Inter. J. Theor. Phys. 28(1989)967) FS(W. Fischler and L. Susskind, hep-th/9806039) Comparison of BEB and FS-HEB • Around the Plank time, they appear to be saturated,which could justify the initial “low” entropy value. +
Questions still exit • For collapsing universe, FS entropy bound fails • For universes with negative cosmological constants,FS bound fails (W. Fischler and L. Susskind, hep-th/9806039; N. Kaloper and A. Linde, PRD60(1999)103509; B. Wang and E. Abdalla, PLB471(2000)346)
Hubble Entropy Bound (HB) - Hubble radius - number of Hubble-size regions within the volume - maximum entropy of each Hubble-size region Relation among HB, FS, BEB A possible relation between the FS, HB and a generalized second law of thermodynamics (GSL) has been discussed. (R. Brustein, PRL84(2000)2072; B. Wang and E. Abdalla, PLB466(1999)122, PLB471(2000)346)
Validity of BEB and HB limited self-gravity, strong self-gravity, Friedmann equation For Holographic Bekenstein-Hawking entropy of a universe-sized black hole
Relation among • Substituting • The relation can be written as • This is very similar to the 2D Cardy formula • At the turning point between the limited self-gravity and • strong self-gravity • BEB and HB have been unified • Friedmann equation corresponds to the generalized Cardyformula • (E. Verlinde, hep-th/0008140; B. Wang, E. Abdalla, • PLB503(2001)394)
Inhomogeneous Cosmologies • Pietronero’s (1987) case that luminous large-scale matterdistribution follows a fractal pattern has started a sharpcontroversy in the literature. • CfA1 redshift survey (de Lapparent, Geller & Huchra, 1986,1988) was the first to reveal structures such as filamentsand voids on scales where a random distribution of matterwas expected. • Do observations of large-scale galaxy distribution supportor dismiss a fractal pattern? How inhomogeneous ismatter distribution? • Relativistic aspects of cosmological models. Any relativisticeffect on observations? • (L. Pietronero and col.)
The real reason, though, for our adherence here to the Cosmological Principle is not that it is surely correct, but rather, that it allows us to make use of the extremely limited data provided to cosmology by observational astronomy. … If the data will not fit into this framework, we shall be able to conclude that either the Cosmological Principle or the Principle of Equivalence is wrong. Nothing could be more interesting. Weinberg, 1972
Description of the Inhomogeneous Universe Relativistic Model Let us start with the inhomogeneous spherically symmetric metric as: where where (E. Abdalla, R. Mohayaee, PRD59(1999)084014)
Lemaitre-Tolman-Bondi model • In normalized comoving coordinates the metric of the • parabolic LTB model is • where , and • Characteristics of the realistic model: • Spherical symmetry • Describing a fractal distribution of galaxies, not refering toeither initial of final moment
Our proposal of a holographic principle in inhomogeneous cosmology The entropy inside the apparent horizon can never exceed the area of the apparent horizon in Plank units.
Defining Apparent Horizon to the aerial radius, with the result is the physical apparent horizon, denotes the proper apparent horizon. Fractal behavior in parabolic models have been found by Ribeiro (APJ1992). They are Where and around 0.65 and around 50 are required to obtain fractal solutions. Model 1: Model 2:
Define the local entropy density Standard big-bang cosmology: When a particle becomesnonrelativistic and disappears, its entropy is transferred toother relativistic particle species still present in the thermalplasma. Photons and neutrinos share the entropy of the universe. Reasonable suppose: Entropy of the universe is mainlyproduced before the dust-filled era. +
First Law of thermodynamics, . • Considering that in the expansion of the universe, • The radiation always has the property of black body • Conservation of the number density of the photon • We have: , the expression for the redshift is • We obtain the relation .
The local entropy density in the inhomogeneous case can be expressed as The total entropy measured in the comoving space inside the apparent horizon is For homogeneous dust universe the local entropy density is only a function of proportional to , the consistent total entropy value (B. Wang, E. Abdalla and T. Osada, PRL85(2000)5507)
Fig. 1: Relation between and with different at thebeginning of the dust-filled universe when .
We now face the question: • the holographic principle has to be challenged • it can be used to select a physically acceptablemodel • We prefer the second, more constructive, alternative.
Fig. 2: Inhomogeneous models which can accommodatereasonable entropy to meet the present observable value.
Fig. 3: Choosing parameters in order to meet the entropyvalue in the present observable universe.
Conclusions • Possible to modelize highly inhomogeneousstructures • Entropy constraints can give valuable informationthrough the holographic principle
The number of e-foldings during inflation Number of e-folds: • Horizon problem, flatness problem & entropy problem • Relate to the slow roll parameters and fluctuations prediction of inflation
The number of e-foldings during inflation The existence of an upper bound for the number of e-foldings has been discussed. In general it is model dependent. The bound has been obtained in some very simple cosmological settings, while it is still difficult to be obtained in nonstandard models. Using the holographic principle, the consideration of physical details connected to the universe evolution can be avoided. We have obtained the upper bound for the number of e-foldings for a standard FRW universe as well as non-standard cosmology based on the brane inspired idea of Randall and Sundrum models.
Holographic Principle Motivated by the well-known example of black hole entropy, an influential holographic principle has put forward, suggesting that microscopic degrees of freedom that build up the gravitational dynamics actually reside on the boundary of space-time. This principle developed to the Maldacena's conjecture on AdS/CFT correspondence and further very important consequences, such as Witten's identification of the entropy, energy and temperature of CFT at high temperatures with the entropy, mass and Hawking temperature of the AdS black hole.
Cosmic Holography We thus seek at a description of the powerful holographic principle in cosmological settings, where its testing is subtle. The question of holography therein: for flat and open FLRW universes the area of the particle horizon should bound the entropy on the backward-looking light cone.
Verlinde-Cardy formula FLRW universe filled with CFT with a dual AdS description has been done by Verlinde, revealing that when a universe-sized black hole can be formed, an interesting and surprising correspondence appears between entropy of CFT and Friedmann equation governing the radiation dominated closed FLRW universes. Generalizing Verlinde's discussion to a broader class of universes including a cosmological constant: matching of Friedmann equation to Cardy formula holds for de Sitter closed and AdS flat universes. However for the remaining de Sitter and AdS universes, the argument fails due to breaking down of the general philosophy of the holographic principle. In high dimensions, various other aspects of Verlinde's proposal have also been investigated in a number of works.
Verlinde-Cardy formula in Brane Cosmology Further light on the correspondence between Friedmann equation and Cardy formula has been shed from a Randall-Sundrum. CFT dominated universe as a co-dimension one brane with fine-tuned tension in a background of an AdS black hole, Savonije and Verlinde found the correspondence between Friedmann equation and Cardy formula for the entropy of CFT when the brane crosses the black hole horizon. Confirmed by studying a brane-universe filled with radiation and stiff-matter, quantum-induced brane worlds and radially infalling brane. The discovered relation between Friedmann equation and Cardy formula for the entropy shed significant light on the meaning of the holographic principle in a cosmological setting. The general proof for this correspondence for all CFTs is still difficult at the moment.
The number of e-foldings from holography Our motivation here is the use of the correspondence between the CFT gas and the Friedmann equation establishing an upper bound for the number of e-foldings during inflation. Recently, Banks and Fischler have considered the problem of the number of e-foldings in a universe displaying an asymptotic de Sitter phase, as our own. As a result the number of e-foldings is not larger than 65/85 depending on the type of matter considered.
The number of e-foldings from holography Here we reconsider the problem from the point of view of the entropy content of the Universe, and considering the correspondence between the Friedmann equation and Cardy formula in Brane Universes.
Brane cosmology Metric: We consider a bulk metric defined by and L is the curvature radius of AdS spacetime. k takes the values 0, -1, +1 corresponding to flat, open and closed geometrics, and is the corresponding metric on the unit three dimensional sections.
Brane cosmology Black hole horizon: The relation between the parameter m and the Arnowitt-Deser-Misner (ADM) mass of the five dimensional black hole M is is the volume of the unit 3 sphere.
Brane cosmology Metric on the brane: Here, the location and the metric on the boundary are time dependent. We can choose the brane time such that The metric on the brane is given by
CFT on the brane The Conformal Field Theory lives on the brane, which is the boundary of the AdS hole. The energy for a CFT on a sphere with volume is given by The density of the CFT energy can be expressed as
Entropy The entropy of the CFT on the brane is equal to the Bekenstein-Hawking entropy of the AdS black hole The entropy density of the CFT on the brane is
Friedmann Equation From the matching conditions we find now the cosmological equations in the brane, is the critical brane tension. Taking the Friedmann Eq. reduces to the Friedmann equation of CFT radiation dominated brane universe without cosmological constant. If the brane-world is a de Sitter universe or AdS universe, respectively.
Friedmann Equation Using the Friedmann equation can be written in the form is the effective positive cosmological constant in four dimensions. Using Friedmann equation becomes which corresponds to the movement of a mechanical nonrelativistic particle in a given potential.
Entropy Bound For a closed universe there is a critical value for which the solution extends to infinity (no big crunch) The entropy in such a universe can be rewriten as at the end of inflation. We take to be the energy density during inflation, that is,
Upper bound on the number of e-foldings Scale factor at the exit of inflation leads to the value , where corresponds to the apparent horizon duringinflation, and we obtain We get where we used the usual values
Brane corrections to the Friedmann equation Let us consider now very high energy brane corrections to the Friedmann equation. From the Darmois-Israel conditions we find where l is the brane tension and in the very high energy limit the term dominates. Within the high-energy regime, the expansion laws corresponding to matter and radiation domination are slower than in the standard cosmology. Slower expansion rates lead to a larger value of the number of e-foldings. However, the full calculation has not been obtained due to the lack of knowledge of this high-energy regime.
CFT energy density and entropy density relation The energy density of the CFT and the entropy density are related as follows, Substitute in the Friedmann equation as before, leading to a bound for the entropy, as well as a bound for the scale factor,
Upper bound on the number of e-foldings The era when the quadratic energy density is important. The brane tension is required to be bounded by and then the number of e-foldings is where is taken. The number of e-foldings obtained is bigger than the value in standard FRW cosmology, which is consistent with the argument of Liddle et al.
Upper bound on the number of e-foldings In summary: • we have derived the upper limit for the number of e-foldings based upon the arguments relating Friedmann equation and Cardy formula. • For the standard FRW universe our result is in good agreement with Literatures. • For the brane inspired cosmology in four dimensions we obtained a larger bound. Considering such a high energy context, the expansion laws are slower than in the standard cosmology, and our result can again be considered to be consistent with the known argument. • The interesting point here is that using the holographic point of view, we can avoid a complicated physics during the universe evolution and give a reasonable value for the upper bound of the number of e-foldings.
Supersymmetric inflationary model Besides the standard model, supersymmetry has been considered both as a blessing and as a curse for inflationary model building. • It is a blessing, primarily because it allows one to have very flat potential, as well as to fine-tune any parameters at the tree level. Moreover it seems more natural than the non-symmetric theories. • It is a curse, because during inflation one needs to consider supergravity, where usually all scalar fields have too big masses to support inflation. • Exceptions: The N=1 generic D-term inflation The N=1 supersymmetric F-term inflation avoids the general problem of inflation in supergravity.