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BEGIN:VEVENT
DTSTAMP:20261021T002836Z
LAST-MODIFIED:20121116T203436Z
DTSTART:20100426T163000Z
DTEND:20100426T173000Z
UID:event591@bu.edu
URL:http://physics.bu.edu/internal/events/show/591
SUMMARY:Cosmological Non-Linearities as an Effective Fluid
DESCRIPTION:Featuring Leonardo Senatore\, Harvard University \n\nPart of th
	e HET Seminar Series.\n\nAbstract\n\n The universe is smooth on large scale
	s but very inhomogeneous on small\n scales. Why is the spacetime on large s
	cales modeled to a good\n approximation by the Friedmann equations? Are we 
	sure that small-scale\n non-linearities do not induce a large backreaction?
	 Related to this\, what\n is the effective theory that describes the univer
	se on large scales? In\n this talk I will show some progress in addressing 
	these questions. I will\n show that the effective theory for the long-wavel
	ength universe behaves as\n a viscous fluid coupled to gravity: integrating
	 out short-wavelength\n perturbations renormalizes the homogeneous backgrou
	nd and introduces\n dissipative dynamics into the evolution of long-wavelen
	gth perturbations.\n The effective fluid has small perturbations and is cha
	racterized by a few\n parameters like an equation of state\, a sound speed 
	and a viscosity\n parameter. These parameters can be matched to numerical s
	imulations or\n fitted from observations. We find that the backreaction of 
	small-scale\n non-linearities is very small. Moreover\, we prove that viria
	lized scales\n decouple completely from the large-scale dynamics\, at all o
	rders in the\n post-Newtonian expansion. We propose that our effective theo
	ry be used to\n formulate a well-defined and controlled alternative to conv
	entional\n perturbation theory\, and we discuss possible observational appl
	ications.
LOCATION: \, \, 
STATUS:CONFIRMED
CLASS:PUBLIC
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