PROCEEDINGS Nuclear Physics B (Proc. Suppl.) 35 (1994) 28-43 SUPPLEMENTS North-Holland Baryogenesis A.D. Dolgov 1 The Randall Laboratory of Physics, University of Michigan, Ann Arbor, MI 48109-1120 Abstract: A review on the present state of the baryogenesis is given with an emphasis on electroweak baryogenesis. Technical details of the numerous models considered in the literature are not elaborated but unresolved problems of the isssue are considered. Different logically possible alternatives of the electroweak scenarios are presented. A possible impact of baryogenesis on the universe structure formation is discussed. Baryogenesis is a process of generation of antibaryons, No >> NB. The magnitude of an excess of baryons over antibaryons of the asymmetry is characterized by the which presumably took place at an early ratio of the baryonic number density to the stage of the Universe evolution. Two ques- number density of photons in cosmic mi- tions immediately arise in this connec- crowave background radiation: tions: first, why do we need that and, sec- ond, if baryogenesis is obligatory or one t3 = NB/N.~ = 10 -9- 10 -10 (1) can make the observed Universe without it and the existence of baryogenesis at an This small number means in particular early stage (or stages) is only one of several that the size of the charge asymmetry possible alternatives in cosmology. In my (which is practically 100% now) was tiny opinion baryogenesis is not only possible at high temperatures, T > AQCD ,~ and natural in the frameworks of modern 100 MeV. At these temperatures an- physics but is also necessary for the cre- tibaryons were practically equally abun- ation of the observed Universe at least at dant in the primeval plasma and corre- the same level as inflation. spondingly (NB -- Nt~ / ( NB + NB ) -.~ ~ << 1. Still this number, though very small, is not The idea of baryogenesis emerged from the easy to obtain and the main goal of the- observations that the Universe at some oretical models is to get this number as distance scale lo around us is practi- large as possible. cally 100% charge asymmetric with baryon number density very much exceeding that There are three important problems re- lated to the scale of the asymmetry IB: 1 Permanent address: ITEP, 113259, Moscow, Russia. - 1. What is the magnitude of IB? Is it 0920-5632/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved. SSDI 0920-5632(94)00428-X A.D. Dolgov/ Baryogenesis 29 infinite or, what is practically the same, formulated by Sakharov are the following: larger that the present-day horizon, IB > lu '~ 101° years? May it be rather small, - 1. Baryonic charge nonconservation. say, like a few × 10 Mpc? In the first case - 2. Asymmetry in particle-antiparticle the whole Universe or at least the vis- interactions (breaking of C- and CP- ible part is baryon dominated while in invariance). the second case there may be a consider- - 3. Deviation from thermal equilib- able amount of antibaryons which can be rium. in principle observed by their interaction with matter on the boundaries. Still since It can be shown that neither of these condi- the distance is fairly large the gamma-flux tions are obligatory (see ref. [4]) but coun- from the annihilation would be sufficiently terexample are rather exotic. A nice fea- low. ture of these three conditions is that they - 2. May the Universe be charge asym- are perfectly natural in the frameworks metric only in our neighbourhood, never of the present-day particle physics. Bary- mind how large it is (even larger than the onic charge nonconservation, which was horizon), and be charge symmetric as a the most problematic 25 years ago, now is whole? The last possibility is aesthetically predicted by grand unification models and appealing since particle-antiparticle sym- what's more by the standard electroweak metry is restored on large. theory. Unfortunately these are only theo- - 3. Is the amplitude of the asymmetry retical arguments and the proton remains a constant or may it be a function of stable despite very strong efforts to dis- space points ~ = ~(x, y, z)? The last case cover its decays. The only "experimental" corresponds to the so called isocurvature evidence in favor of baryonic charge non- density fluctuations which may be very in- conservation is given now by cosmology. teresting for the structure formation in low On the contrary C- and CP-violation are Universe. observed experimentally in particle physics and we may be sure that particles and an- The idea that the dominance of baryons tiparticles are indeed different. Still theo- over antibaryons can be explained dynam- retically this phenomenon is not well un- ically was first proposed by Sakharov [1] in derstood: there are many models for CP- 1967. Before it was a common belief that a violation and we do not yet know which nonzero baryonic charge of the Universe is one is true. As for deviation from ther- a result of mysterious initial conditions. At mal equilibrium it is provided by the uni- the present day there are several hundred verse expansion and always exists for mas- papers on the subject discussing different sive particles with the relative magnitude possible scenarios of the generation of the of the order (m2/T2)(H/F) where T is the baryon asymmetry of the Universe. The temperature of the primeval plasma, H is history of the problem as well as long lists the Hubble parameter characterizing the of references can be found in the review pa- expansion rate, and F is the reaction rate. pers [2-5]. Three very well known by now This expression is valid for m < T and conditions of the baryogenesis which were is typically rather small. For m > T the 30 A.D. Dolgov/Baryogenesis contribution of the particles with the mass tion of state p = -p corresponds to anti- m is usually exponentially suppressed so gravitating medium creating expansion. in both cases deviations from equilibrium are small. This smallness is not crucial for Of course inflationary models have their scenarios of baryogenesis at grand unifi- own problems like very small strength of cation scale but may be very important the inflaton interactions and the absence for lower temperatures. Fortunately there of a natural inflationary scenario in the is another way to break the equilibrium frameworks of the simplest gauge theo- by the first order phase transition. In that ries of particle interactions which is an case one may expect a low energy baryo- argument against inflation. On the other genesis, T << TOUT ~ 1016 GeV. Anyhow hand the prediction of inflationary mod- some deviations from thermal equilibrium els of approximately flat spectrum of den- always exist in the cosmological plasma sity perturbations is in a reasonable agree- and this provides the third necessary con- ment with the COBE data (see the talk by dition for baryogenesis. J.Silk at this Conference). Slightly tilted spectrum of density fluctuations may bet- We see that baryogenesis might happen in ter describe the Universe structure forma- the course of the Universe evolution and tion and fortunately there exist inflation- now I would like to argue that it indeed ary models which can give this prediction took place. The crucial point is that in- (see e.g. [6]). Another quantitative predic- flation is impossible without baryogenesis. tion of inflation that the density parame- One may argue that the existence of in- ter f~ is most probably equal to one may flation could also be questioned. Strictly be in agreement with observations but the speaking this is true since we do not have latter are very inaccurate and one cannot rigorous proof that the Universe, as we make a decisive conclusion here. Plenty of see it, cannot be created without infla- people would be happy if f~ is considerably tion. Moreover this proof can never be pre- smaller than one. In that case we may not sented. However inflation is the only sce- need nonbaryonic dark matter and in view nario which solves in a simple way many of the recent announcements by the ex- cosmological problems which cannot be perimental groups EROS and MACHOS of addressed in any other known cosmological possible microlensing events (see the talks scenario. Among them are the problems of by A. Milsztajn and B. Sadoulet at this Conference) one may think that all the - 1) flatness; the Universe should be fiat dark matter in the Universe is baryonic. with the accuracy 10 -~5 during primordial The claim that there is some baryonic dark nucleosynthesis, matter in the Universe is supported by the -2) horizon, homogeneity, and iso- primordial nucleosynthesis theory which tropy, gives ~s ~ O.05(H/5Okm/sec/Mpc) -2 [7] - 3) generation of the primordial den- while the contribution of the visible bary- sity fluctuations, onic matter is FtB ~ 0.01. However purely - 4) initial push which gave rise to the baryonic universe encounters serious diffi- Universe expansion; the inflationary equa- culties in large scale structure formation AID. Dolgov/Baryogenesis 31 especially because of very small fluctua- SU(3) x SU(2) x U(1)-model (MSM). A tions of the microwave background tem- strong indication of the validity of the perature. From this point of view nonbary- grand unification is the crossing of all three onic dark matter and large (close to 1) fl gauge coupling constants of supersymmet- are very desirable. Taken together with the ric extension of MSM at the same point nice inflationary solution of the basic cos- near EGUT = 1016 GeV. It is rather diffi- mological problems this gives a very strong cult to believe that there are no new parti- argument in favor of inflationary scenario.