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Ann Elizabeth Nelson was a particle physicist and professor of physics in the Particle Theory Group at the University of Washington from 1994 until her death. Nelson received a Guggenheim Fellowship in 2004, and she was elected to the American Academy of Arts and Sciences in 2011 and the National Academy of Sciences in 2012. She was a recipient of the 2018 J. J. Sakurai Prize for Theoretical Particle Physics, presented annually by the American Physical Society and considered one of the most prestigious prizes in physics.
In particle physics and string theory (M-theory), the ADD model, also known as the model with large extra dimensions (LED), is a model framework that attempts to solve the hierarchy problem. The model tries to explain this problem by postulating that our universe, with its four dimensions, exists on a membrane in a higher dimensional space. It is then suggested that the other forces of nature operate within this membrane and its four dimensions, while the hypothetical gravity-bearing particle graviton can propagate across the extra dimensions. This would explain why gravity is very weak compared to the other fundamental forces. The size of the dimensions in ADD is around the order of the TeV scale, which results in it being experimentally probeable by current colliders, unlike many exotic extra dimensional hypotheses that have the relevant size around the Planck scale.
In physical cosmology and astronomy, dark energy is an unknown form of energy that affects the universe on the largest scales. The first observational evidence for its existence came from measurements of supernovas, which showed that the universe's expansion is accelerating. Understanding the universe's evolution requires knowledge of its starting conditions and composition. Before these observations, scientists thought that all forms of matter and energy in the universe would only cause the expansion to slow down over time. Measurements of the cosmic microwave background (CMB) suggest the universe began in a hot Big Bang, from which general relativity explains its evolution and the subsequent large-scale motion. Without introducing a new form of energy, there was no way to explain an accelerating expansion of the universe. Since the 1990s, dark energy has been the most accepted premise to account for the accelerated expansion. As of 2021, there are active areas of cosmology research to understand the fundamental nature of dark energy. Assuming that the lambda-CDM model of cosmology is correct, as of 2013, the best current measurements indicate that dark energy contributes 68% of the total energy in the present-day observable universe. The mass–energy of dark matter and ordinary (baryonic) matter contributes 26% and 5%, respectively, and other components such as neutrinos and photons contribute a very small amount. Dark energy's density is very low: 6×10−10 J/m3, much less than the density of ordinary matter or dark matter within galaxies. However, it dominates the universe's mass–energy content because it is uniform across space.
Light dark matter, in astronomy and cosmology, are dark matter weakly interacting massive particles (WIMPS) candidates with masses less than 1 GeV. These particles are heavier than warm dark matter and hot dark matter, but are lighter than the traditional forms of cold dark matter, such as Massive Compact Halo Objects (MACHOs). The Lee-Weinberg bound limits the mass of the favored dark matter candidate, WIMPs, that interact via the weak interaction to GeV. This bound arises as follows. The lower the mass of WIMPs is, the lower the annihilation cross section, which is of the order , where m is the WIMP mass and M the mass of the Z-boson. This means that low mass WIMPs, which would be abundantly produced in the early universe, freeze out much earlier and thus at a higher temperature, than higher mass WIMPs. This leads to a higher relic WIMP density. If the mass is lower than GeV the WIMP relic density would overclose the universe.
In astrophysics and cosmology scalar field dark matter is a classical, minimally coupled, scalar field postulated to account for the inferred dark matter.
References
- ↑ "New Theory Links Neutrino's Slight Mass To Accelerating Universe Expansion". www.sciencedaily.com. Archived from the original on 13 May 2008. Retrieved 2008-06-05.
- ↑ Kaplan, D.; Nelson, A.; Weiner, N. (2004). "Neutrino Oscillations as a Probe of Dark Energy". Physical Review Letters . 93 (9): 091801. arXiv: hep-ph/0401099 . Bibcode:2004PhRvL..93i1801K. doi:10.1103/PhysRevLett.93.091801. PMID 15447091. S2CID 8861027.
- ↑ Fardon, R.; Nelson, A.; Weiner, N. (2004). "Dark energy from mass varying neutrinos". Journal of Cosmology and Astroparticle Physics . 2004 (10): 005. arXiv: astro-ph/0309800 . Bibcode:2004JCAP...10..005F. doi:10.1088/1475-7516/2004/10/005. S2CID 250827123.
- ↑ Stricherz, Vince (2004-07-27). "New theory links neutrino's slight mass to accelerating universe expansion". Archived from the original on 2004-08-22. Retrieved 2023-09-06.
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