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Home , Baryogenesis, Big Bang, Big Bang nucleosynthesis

The Himalayan , Vol.1, No.1, May 2010 The Foreground of Ravi Karki Department of Physic, PN Campus, Pokhara, Email: [email protected] value makes little difference to the overall picture. In physical , Without major changes to the Big Bang (or primordial nucleosynthesis, BBN) refers to the itself, BBN will result in abundances of about production of nuclei other than those of H-1 (i.e. 75% of H-1, about 25% -4, about 0.01% of the normal, of , whose nuclei , trace (on the order of 10-10) amounts of consist of a single each) during the early and , and no other heavy elements. phases of the . Primordial nucleosynthesis (Traces of have been found in some young took place just a few minutes after the Big Bang and , giving rise to the question that some boron, is believed to be responsible for the formation of a not really predicted by the theory, might have been heavier isotope of hydrogen known as deuterium produced in the Big Bang. The question is not (H-2 or D), the helium He-3 and He-4, presently resolved. That the observed abundances and the lithium isotopes Li-6 and Li-7. In addition in the universe are generally consistent with these to these stable nuclei some unstable, or radioactive, abundance numbers is considered strong evidence isotopes were also produced during primordial for .In this fi eld it is customary nucleosynthesis: or H-3; beryllium-7 (Be-7), to quote percentages by mass, so that 25% helium-4 and beryllium-8 (Be-8). These unstable isotopes means that helium-4 account for 25% of the either decayed or fused with other nuclei to make mass, but only about 8% of the atoms would be one of the stable isotopes. helium-4 atoms. Characteristics of Big Bang nucleosynthesis Sequence of Big Bang nucleosynthesis There are two important characteristics of Big Bang Big Bang nucleosynthesis begins about three minutes nucleosynthesis (BBN): after the Big Bang, when the universe has cooled It lasted for only about seventeen minutes (during down suffi ciently to form stable and , the from 3 to about 20 minutes from the after . The relative abundances of these beginning of expansion after that, the particles follow from simple thermodynamical and of the universe fell below arguments, combined with the way that the mean that which is required for . The brevity temperature of the universe changes over (if the of BBN is important because it prevented elements reactions needed to reach the thermodynamically heavier than beryllium from forming while at the favoured equilibrium values are too slow compared same time allowing unburned light elements, such to the temperature change brought about by the as deuterium, to exist. expansion, abundances will remain at some specifi c non-equilibrium value). Combining thermodynamics It was widespread, encompassing the entire and the changes brought about by cosmic expansion, . The key parameter which one can calculate the fraction of protons and neutrons allows one to calculate the effects of BBN is the based on the temperature at this point. The answer number of per . This parameter is that there are about seven protons for every corresponds to the temperature and density of the at the beginning of nucleogenesis, a ratio early universe and allows one to determine the that would remain stable even after nucleogenesis conditions under which nuclear fusion occurs. is over. This fraction is in favour of protons initially From this we can derive elemental abundances. primarily because lower mass of the proton favors Although the baryon per ratio is important their production. Free neutrons also decay to protons in determining elemental abundances, the precise with a half- of about 15 minutes, and this time- 79 Karki/The Foreground... scale is too short to affect the number of neutrons nucleosynthesis was much less than the observed over the period in which BBN took place, primarily mass of the universe based on calculations of the because most of the free neutrons had already been expansion rate. This puzzle was resolved in large absorbed in the fi rst 3 minutes of nucleogenesis-- a part by postulating the existence of dark . time too short for a signifi cant fraction of them to decay to protons. Heavy elements Big Bang nucleosynthesis produced no elements One feature of BBN is that the physical laws and heavier than beryllium, due to a bottleneck due to the constants that govern the behavior of matter at absence of a stable nucleus with 8 or 5 . In these are very well understood, and hence stars, the bottleneck is passed by triple collisions of BBN lacks some of the speculative uncertainties helium-4 nuclei, producing (the triple-alpha that characterize earlier periods in the life of the process). However, this process is very slow, taking universe. Another feature is that the process of tens of thousands of to convert a signifi cant nucleosynthesis is determined by conditions at the amount of helium to carbon in stars, and therefore start of this phase of the life of the universe, making it made a negligible contribution in the minutes what happens before irrelevant. following the Big Bang. As the universe expands, it cools. Free neutrons and Helium-4 protons are less stable than helium nuclei, and the protons and neutrons have a strong tendency to form Big Bang nucleosynthesis predicts a primordial helium-4. However, forming helium-4 requires the abundance of about 25% helium-4 by mass, intermediate step of forming deuterium. At the time irrespective of the initial conditions of the universe. at which nucleosynthesis occurs, the temperature As long as the universe was hot enough for protons is high enough for the mean per particle to and neutrons to transform into each other easily, their be greater than the binding energy of deuterium; ratio, determined solely by their relative , therefore any deuterium that is formed is immediately was about 1 neutron to 7 protons (allowing for some destroyed (a situation known as the deuterium decay of neutrons into protons). Once it was cool bottleneck). Hence, the formation of helium-4 is enough, the neutrons quickly bound with an equal delayed until the universe becomes cool enough number of protons to form helium-4. Helium-4 is to form deuterium (at about T = 0.1 MeV), when very stable and neither decays nor combines easily there is a sudden burst of element formation. Shortly to form heavier nuclei. So out of every 16 nucleons thereafter, at twenty minutes after the Big Bang, the (2 neutrons and 14 protons), 4 of these (25%) universe becomes too cool for any nuclear fusion to combined into one helium-4 nucleus. One analogy occur. At this point, the elemental abundances are is to think of helium-4 as ash, and the amount of ash fi xed, and only change as some of the radioactive that one forms when one completely burns a piece of products of BBN (such as tritium) decay. wood is insensitive to how one burns it.

History of Big Bang nucleosynthesis theory The helium-4 abundance is important because there is far more helium-4 in the universe than can be The history of Big Bang nucleosynthesis began with explained by . In addition, it the calculations of Ralph Alpher and provides an important test for the Big Bang theory. in the 1940s. Together with they would If the observed helium abundance is much different publish the seminal Alpher-Bethe-Gamow paper from 25%, then this would pose a serious challenge outlining the theory of light-element production in to the theory. This would particularly be the case if the early universe. the early helium-4 abundance was much smaller than 25% because it is hard to destroy helium-4. For a few During the 1970s, there was a major puzzle in that years during the mid-1990s, observations suggested the density of as calculated by Big Bang that this might be the case, causing astrophysicists 80 The Himalayan Physics, Vol.1, No.1, May 2010 to talk about a Big Bang nucleosynthetic crisis, but not consist mostly of baryons, and that non-baryonic further observations were consistent with the Big matter (also known as ) makes up most Bang theory. of the matter mass of the universe. This explanation is also consistent with calculations that show that a Deuterium universe made mostly of protons and neutrons would be far more clumpy than is observed. Deuterium is in some ways the opposite of helium-4 in that while helium-4 is very stable and very It is very hard to come up with another process that diffi cult to destroy, deuterium is only marginally would produce deuterium via nuclear fusion. What stable and easy to destroy. Because helium-4 is this process would require is that the temperature very stable, there is a strong tendency on the part of be hot enough to produce deuterium, but not hot two deuterium nuclei to combine to form helium-4. enough to produce helium-4, and that this process The only reason BBN does not convert all of the immediately cools down to non-nuclear deuterium in the universe to helium-4 is that the after no more than a few minutes. Also, it is expansion of the universe cooled the universe necessary for the deuterium to be swept away before and cut this conversion short before it could be it reoccurs. completed. One consequence of this is that unlike helium-4, the amount of deuterium is very sensitive Producing deuterium by fi ssion is also diffi cult. to initial conditions. The denser the universe is, the The problem here again is that deuterium is very more deuterium gets converted to helium-4 before subject to nuclear processes, and that collisions time runs out, and the less deuterium remains. between atomic nuclei are likely to result either in the absorption of the nuclei, or in the release of There are no known post-Big Bang processes which free neutrons or alpha particles. During the 1970s, would produce signifi cant amounts of deuterium. attempts were made to use to Hence observations about deuterium abundance produce deuterium. These attempts failed to produce suggest that the universe is not infi nitely old, which deuterium, but did unexpectedly produce other light is in accordance with the Big Bang theory. elements. During the 1970s, there were major efforts to fi nd Observational Tests and Status of Big Bang processes that could produce deuterium, which nucleosynthesis turned out to be a way of producing isotopes The theory of BBN gives a detailed mathematical other than deuterium. The problem was that while description of the production of the light “elements” the concentration of deuterium in the universe is deuterium, helium-3, helium-4, and lithium-7. consistent with the Big Bang model as a whole, it is Specifi cally, the theory yields precise quantitative too high to be consistent with a model that presumes predictions for the mixture of these elements, that is, that most of the universe consists of protons and the primordial abundances. neutrons. If one assumes that all of the universe consists of protons and neutrons, the density of the In order to test these predictions, it is necessary to universe is such that much of the currently observed reconstruct the primordial abundances as faithfully deuterium would have been burned into helium-4. as possible, for instance by observing astronomical objects in which very little stellar nucleosynthesis This inconsistency between observations of has taken place (such as certain dwarf ) or deuterium and observations of the expansion rate of by observing objects that are very far away, and thus the universe led to a large effort to fi nd processes can be seen in a very early stage of their that could produce deuterium. After a decade of (such as distant ). effort, the consensus was that these processes are unlikely, and the standard explanation now used for As noted above, in the standard picture of BBN, all of the abundance of deuterium is that the universe does the light element abundances depend on the amount 81 Karki/The Foreground... of ordinary matter (baryons) relative to resolve inconsistencies between BBN predictions (photons). Since the universe is homogeneous, it and observations. This has proved to be of limited has one unique value of the baryon-to-photon ratio. usefulness in that the inconsistencies were resolved For a long time, this meant that to test BBN theory by better observations, and in most cases trying to against observations one had to ask: can all of the change BBN resulted in abundances that were more light element observations be explained with a inconsistent with observations rather than less. The single value of the baryon-to-photon ratio? Or more , which is largely the focus of non-standard precisely, allowing for the fi nite precision of both BBN in the early 21st century, is to use BBN to the predictions and the observations, one asks: is place limits on unknown or speculative physics. there some range of baryon-to-photon values which For example, standard BBN assumes that no exotic can account for all of the observations? hypothetical particles were involved in BBN. One can insert a hypothetical particle (such as a massive More recently, the question has changed: Precision ) and see what has to happen before BBN observations of the cosmic background predicts abundances which are very different from radiation with the Wilkinson Microwave observations. This has been usefully done to put Probe (WMAP) give an independent value for the limits on the mass of a stable . baryon-to-photon ratio. Using this value, are the BBN predictions for the abundances of light elements in agreement with the observations?

The measurement of helium-4 indicates good agreement, and yet better agreement for helium-3. But for lithium-7, there is a signifi cant discrepancy between BBN and WMAP, and the abundance derived from Population II stars. The discrepancy is a factor of 2.4―4.3. and is considered a problem for the original models, that have resulted in revised calculations of the standard BBN based on new nuclear data, and to various reevaluation proposals for primordial proton-proton nuclear reactions, especially the intensities of 7Be(n,p)7Li versus 7Be(d,p)8Be. Non-standard Big Bang nucleosynthesis In addition to the standard BBN scenario there are numerous non-standard BBN scenarios. These should not be confused with non-standard cosmology: a non-standard BBN scenario assumes that the Big Bang occurred, but inserts additional physics in order to see how this affects elemental abundances. These pieces of additional physics include relaxing or removing the assumption of , or inserting new particles such as massive .

There have been, and continue to be, various reasons for researching non-standard BBN. The fi rst, which is largely of historical interest, is to

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