Lecture 12 Flashcards
what is dark energy
the acceleration of the expansion of the universe is caused by something given the name dark energy
what would we expect the universes expansion to be without dark energy
would expect gravity from ordinary and dark matter pulling the universe together slowing down the expansion, not accelerating it
what does the Hubble parameter tell us now?
current expansion rate
age of the universe with and without acceleration (or empty)
mass only - 9-10 billion years
acceleration(or empty) - 13-14 billion
what are the oldest detectable stars and what does this tell us?
12-13 billion years, so universe must have acceleration
what are type 1a supernovae
a massive star pulls matter off a partner star until it becomes unstable and explodes with predictable brightness
what do 1a supernovae tell us about the age of the universe
high-z supernovae are further(fainter) than predicted in a non-accelerating universe so the universe has expanded more than predicted
what is the cosmological constant
simplest model for dark energy, in CC empty space has an energy density of pure vaccum
how can we have negative gravity?
Einsteins GR allows energy, pressure, momentum and stress to cause gravity which gives us a way to make negative gravity (negative pressure) since we cant have negative mass
what is the current leading explanation for dark energy
mass causes positive pressure, dark energy has negative pressure
since energy in empty space has negative pressure, negative pressure causes negative gravity, which makes the universe accelerate
what is wrong with quantum theory prediction of cosmological constant and what is it
particles flicker in and out of existence and microscopic scales as the energy fields that describe them have a zero point energy
could be dark energy but prediction is wrong by factor of 10^120
what are the issues with dark energy
we have a fine tuning problem, there is a number we expect but there Turns out to be a vastly smaller value
why is the amount of dark matter within a factor of two of the amount of dark energy? this could suggest they are related which we don’t believe them to be
what Is the density of the universe made up of
25.9% dark matter, 4.9% baryonc matter, 69.2% Dark energy
what does the cosmological constant model assume about the location of dark energy
its exactly constant everywhere
what are quintessence models
propose dark energy field varies in space and time
has waves of high and lower values that can travel
how can we test dark energy models
the equation of state - relate pressure and density of dark energy, lots of work to measure the value of w in the equation by seeing its effect on cosmic expansion and clustering
what is the current results of the DESI experiment
the experiment measures galaxy clustering
suggests that w is changing which would rule out cosmological constant
probably some mismatch between models masquerading as dark energy
what could a dark energy alternative be
unknown laws of gravity
