Dark energy
Lecture 27 Wednesday, November 16, 2011 12:30 PM
The threshold density doesn’t add up to 100%, meaning we will expand forever In WMAP we see we are at the threshold density We have an extra component that's just extra energy, which makes the universe expand faster Dark energy is uniformly distributed but has no gravity
Recall about WMAP - The degree of lumpiness tells us the total amount of mass/energy in space - If very smooth then not much mass in space, universe would expand forever - If very clumpy then lots of mass/energy in space; universe expansion would stop and recollapse - There is a "threshold density" of mass/energy below which expansion would continue forever - Visible stars + gas makes up ~4% of this threshold - Indirectly detected dark matter makes up 23% - But WMAP picture says that the total mass/energy density is very nearly = to threshold density! - The remaining 73% is now suggested to be composed of pure energy in space ("dark energy") - This dark energy could be the same kind of energy held within spacetime grid that was in the early inflation epoch, but just at a much lower level
Dark Matter vs. Dark Energy - DM: any type of mass that does not emit light. Detectable through its gravity ○ Several possible kinds of objects in this category ○ Many times as much mass in "Dark" form as in luminous stars and gas - DE: pure energy - does not produce gravity. Even harder to detect evidence for it ○ What is it and how does it show up?
Dark energy - Came from looking at supernovas (incredibly bright), we can see them from 7 billion light years away and is visible for a few weeks - When you can see a supernova you can measure how far away it is (standard candles) - So, if you see one in a very distant galaxy you can use it to measure the distance to that galaxy using Hubble's law - Supernova are good standard candles (Called type 1A) - Alternate SN: white dwarfs with another star, stealing mass can explode (with a critical mass of 4 solar masses) - You observe a patch of the sky and subtract away from this image, type 1A SN release where iron
Discovery - These two methods start disagreeing more and more at larger distances, the supernova method says that the galaxy is farther away than Hubble's Law says -- so something is missing - The effect behaves exactly as if space has some extra energy built into it ○ Investigation of these supernova in distant galaxies thus had a surprising outcome - the discovery of dark energy - This dark energy would be kind of "springiness" built into space. During the inflation era of the big bang, there was lots of it. Is this present-day dark energy a left over bit from the inflationary era? Or some other cause?
Gravity vs. Dark Energy - Gravity from galaxies and dark matter causes expansion to slow down over time - The energy in space itself would cause the acceleration to speed up with time - Which effect is more important? ○ At early times gravity is. (galaxies and mass more densely crowded together.) But as they get farther apart, eventually dark energy acceleration will dominate
Some graphs and math ASTRON 2B03 Page 1
Some graphs and math - The supernova measurements of the expansion rate are now showing that our universe has already passed this stage from slowing down to speeding-up. We are now in an accelerating phase which will continue indefinitely
- How our universe would have evolved
- But using evidence from dark energy we know that this is how has evolved
- So it looks like our universe is not just expanding, but accelerating! (Went from decelerating to accelerating about 5 billion years ago) - If this continues then the night sky will be boring in the distant future! - Lots of cool physics and big questions left to answer (what is DE and what is DM?)
Ideas about what it could be: - Cosmological constant (Einstein's idea which he later discarded) - Something else - all sorts of wacky ideas! - Regardless any fundamental theory combing the quantum and mechanics (like string theory) must explain dark energy
Prospects for cosmology? ASTRON 2B03 Page 2
Prospects for cosmology? -
Lots of cool theoretical astrophysics need to be done Dakr matter requires new particles or new law of gravity Dark energy requires new physics, maybe really wacky physics Both DM and DE will have to also work in theory of quantum gravity like string theory
ASTRON 2B03 Page 3
The threshold density doesn’t add up to 100%, meaning we will expand forever In WMAP we see we are at the threshold density We have an extra component that's just extra energy, which makes the universe expand faster Dark energy is uniformly distributed but has no gravity
Recall about WMAP - The degree of lumpiness tells us the total amount of mass/energy in space - If very smooth then not much mass in space, universe would expand forever - If very clumpy then lots of mass/energy in space; universe expansion would stop and recollapse - There is a "threshold density" of mass/energy below which expansion would continue forever - Visible stars + gas makes up ~4% of this threshold - Indirectly detected dark matter makes up 23% - But WMAP picture says that the total mass/energy density is very nearly = to threshold density! - The remaining 73% is now suggested to be composed of pure energy in space ("dark energy") - This dark energy could be the same kind of energy held within spacetime grid that was in the early inflation epoch, but just at a much lower level
Dark Matter vs. Dark Energy - DM: any type of mass that does not emit light. Detectable through its gravity ○ Several possible kinds of objects in this category ○ Many times as much mass in "Dark" form as in luminous stars and gas - DE: pure energy - does not produce gravity. Even harder to detect evidence for it ○ What is it and how does it show up?
Dark energy - Came from looking at supernovas (incredibly bright), we can see them from 7 billion light years away and is visible for a few weeks - When you can see a supernova you can measure how far away it is (standard candles) - So, if you see one in a very distant galaxy you can use it to measure the distance to that galaxy using Hubble's law - Supernova are good standard candles (Called type 1A) - Alternate SN: white dwarfs with another star, stealing mass can explode (with a critical mass of 4 solar masses) - You observe a patch of the sky and subtract away from this image, type 1A SN release where iron
Discovery - These two methods start disagreeing more and more at larger distances, the supernova method says that the galaxy is farther away than Hubble's Law says -- so something is missing - The effect behaves exactly as if space has some extra energy built into it ○ Investigation of these supernova in distant galaxies thus had a surprising outcome - the discovery of dark energy - This dark energy would be kind of "springiness" built into space. During the inflation era of the big bang, there was lots of it. Is this present-day dark energy a left over bit from the inflationary era? Or some other cause?
Gravity vs. Dark Energy - Gravity from galaxies and dark matter causes expansion to slow down over time - The energy in space itself would cause the acceleration to speed up with time - Which effect is more important? ○ At early times gravity is. (galaxies and mass more densely crowded together.) But as they get farther apart, eventually dark energy acceleration will dominate
Some graphs and math ASTRON 2B03 Page 1
Some graphs and math - The supernova measurements of the expansion rate are now showing that our universe has already passed this stage from slowing down to speeding-up. We are now in an accelerating phase which will continue indefinitely
- How our universe would have evolved
- But using evidence from dark energy we know that this is how has evolved
- So it looks like our universe is not just expanding, but accelerating! (Went from decelerating to accelerating about 5 billion years ago) - If this continues then the night sky will be boring in the distant future! - Lots of cool physics and big questions left to answer (what is DE and what is DM?)
Ideas about what it could be: - Cosmological constant (Einstein's idea which he later discarded) - Something else - all sorts of wacky ideas! - Regardless any fundamental theory combing the quantum and mechanics (like string theory) must explain dark energy
Prospects for cosmology? ASTRON 2B03 Page 2
Prospects for cosmology? -
Lots of cool theoretical astrophysics need to be done Dakr matter requires new particles or new law of gravity Dark energy requires new physics, maybe really wacky physics Both DM and DE will have to also work in theory of quantum gravity like string theory
ASTRON 2B03 Page 3
