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Big Bang - PPT Presentation

Nucleosynthesis The Primordial Lithium Problem Matthew von Hippel 1 Outline What is Big Bang Nucleosyntheis How does it work How can we check it The Primordial Lithium Problem Problem or problem ID: 170901

bang big problem lithium big bang lithium problem universe early nucleosynthesis stars physics bbn density ratio baryon scale reactions observations nuclear cosmic

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Slide1

Big Bang Nucleosynthesis:The Primordial Lithium Problem

Matthew von Hippel

1Slide2

OutlineWhat is Big Bang Nucleosyntheis?How does it work?How can we check it?The Primordial Lithium ProblemProblem or problem?Possibility of New Physics2Slide3

“We are star-stuff” –Carl SaganWell not exactly. Many elements come from stars or supernovae, but the lightest need another originHence, Big Bang Nucleosynthesis (BBN) 3Slide4

Big Bang NucleosynthesisFirst proposed by Ralph Alpher, Hans Bethe, and George Gamow, 1948Bethe wasn’t actually involved in the research, but was added for the alpha-beta-gamma punThey proposed that all elements were formed in the Big Bang. This turns out to be false: there is no stable nucleus of mass number five or eight, which prevents the process from going beyond Lithium.

4Slide5

BBN At a GlanceEinstein’s Equations in a universe with an isotropic distribution of matter and energy give us the Friedmann Equations: Where a(t) is a universal scale factor and k is related to the universe’s curvatureWith these and the equation of state for gas we can write the scale factor in terms of the densityThe early universe was very hot, thus dominated by radiation. Because of this the density near the Big Bang is approximately the radiation density, which thermodynamics lets us relate to the temperaturePut all this together, and we get

5Slide6

The ReactionsFrom this relation, we get the temperature of the early universe, which tells us which reactions take placeLight elements are produced by the following reactions:Inputting 11 key nuclear cross sections, the baryon-photon ratio, and the neutron lifetime, we can predict the relative abundances of Deuterium, Lithium-7, Helium-3, and Helium-4 compared to Hydrogen6Slide7

Cosmic Microwave BackgroundThe angular power spectrum of the Cosmic Microwave Background can give us a value for the baryon-photon ratioThe ratio of heights between odd and even peaks increases with baryon density. Other parameters move the peaks in synchThere were some early discrepancies, but more recent measurements (WMAP) give baryon-photon ratio 6.23±0.17This is then used in the BBN formulas7Slide8

What do we compare it to?To observe whether the ratios calculated by BBN hold, we measure old parts of the universe where fusion has been of limited scope.8

In particular, three areas for Lithium:

Metal-poor halo stars: Li in these stars correlates with Fe, so by taking Fe to zero we get a value for Li in the early universe

Globular clusters: Similar situation

Metal-poor high velocity clouds: Not pursued in detail yet, may offer a check on the above twoSlide9

What do we get?Yellow and Empty curves are observations, Blue is theoryY=He/(H+He)Helium 3 is hard to measure, since most stars burn it, so there is no yellow curveRemaining curves within expected error…except Lithium. Is this a problem?

9Slide10

Problems vs. problemsTo find discrepancies that indicate new physics, we first have to be sure the discrepancy isn’t caused by more prosaic sources of inaccuracy in our measurements/calculations.Essentially, whether it is a Problem that can spark a new theory, or merely a problem with our current calculationWhat else could go wrong?Nuclear Cross Sections: in general, we might have missed some key nuclear reactions that reduce cosmic Lithium. However, the nuclear theory involved is well understood. Those parts that are more poorly understood are constrained by the role they play in models of stars, which end up meaning that corrections here will likely be in the wrong direction.More observations: current observations could be supplemented by unexplored areas(high velocity clouds). However, we will likely still have to use halo stars as a standard measurement, so this might cause dramatic change.

10Slide11

New Physics!If it really is a Problem, not just an issue with our methods, then that means new physics.In general:Today’s Li-7 from primordial Li-7 and Be-7. In the early universe the following reaction would get rid of Be-7: where Li-7 is destroyed by proton reactionsSo to reduce Li, we need more neutrons.WIMP decays an early speculation, but insufficientDecays of GeV scale SUSY particles might bring more neutrons through more complex paths (

Pospelov and Pradler, 2010)

11Slide12

ConclusionBig Bang Nucleosynthesis allows predictions of present-day light element abundances, once we have CMB anisotropy dataThese match observations well, except for LithiumThe missing Lithium is likely a sign of physics beyond the standard model12Slide13

BibliographyCheng, T.-P. (2005). Relativity, Gravitation, and Cosmology: A Basic Introduction. Oxford, Oxford University Press. Cyburt, R. H. and et al. (2008). "An update on the big bang nucleosynthesis prediction for 7 Li: the problem worsens." Journal of Cosmology and Astroparticle Physics 2008(11): 012.Kaplinghat, M. T., Michael S. (2001). "Precision Cosmology and the Density of Baryons in the Universe." Physical Review Letters 86(3): 4. Pospelov, M. a. P., Josef (2010) Metastable GeV-scale particles as a solution to the cosmological lithium problem.

arXiv:1006.4172 Turner, M. S. (1996) Big-bang Nucleosynthesis

: Is the Glass Half Full or Half Empty?

arXiv:astro

-ph/9610158v1

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