Lecture 17: the CMB and BBN As with all course material (including homework, exams), these lecture notes are not be reproduced, redistributed, or sold in any form.
Peering out/back into the Universe As we look out into the Universe, we see the Universe as it was at some time in the past. The farther away, the farther back in time. [Due to expansion of Universe, the more distant light is redshifted more.] t = today, z = 0 d = 0 [not to scale]
Peering out/back into the Universe As we look out into the Universe, we see the Universe as it was at some time in the past. The farther away, the farther back in time. [Due to expansion of Universe, the more distant light is redshifted more.] t = 1 Gyr ago, z = 0.08 d ~ 300 Mpc [nearby] t = today, z = 0 d = 0 [not to scale]
Peering out/back into the Universe As we look out into the Universe, we see the Universe as it was at some time in the past. The farther away, the farther back in time. [Due to expansion of Universe, the more distant light is redshifted more.] t = 12 Gyr ago, z = 4 d ~ 40 Gpc [distant] t = 1 Gyr ago, z = 0.08 d ~ 300 Mpc [nearby] t = today, z = 0 d = 0 [not to scale]
Peering out/back into the Universe As we look out into the Universe, we see the Universe as it was at some time in the past. The farther away, the farther back in time. [Due to expansion of Universe, the more distant light is redshifted more.] e - γ e - γ n γ p t ~ 13.5 Gyr ago, z = 1000 d ~ very distant t = 12 Gyr ago, z = 4 d ~ 40 Gpc [distant] t = 1 Gyr ago, z = 0.08 d ~ 300 Mpc [nearby] t = today, z = 0 d = 0 [not to scale]
Peering out/back into the Universe As we look out into the Universe, we see the Universe as it was at some time in the past. The farther away, the farther back in time. [Due to expansion of Universe, the more distant light is redshifted more.] e - γ e - γ p n γ CMB is a picture of the Universe when light was first able to move freely. t ~ 13.5 Gyr ago, z = 1000 d ~ very distant t = 12 Gyr ago, z = 4 d ~ 40 Gpc [distant] t = 1 Gyr ago, z = 0.08 d ~ 300 Mpc [nearby] t = today, z = 0 d = 0 [not to scale]
time = 0 (big bang) time = 13.8 Gyr (today) X (us) distance
time = 0 (big bang) time = 13.8 Gyr (today) (t, d) or z X (us) distance
time = 0 (big bang) time = 13.8 Gyr (today) (t, d) or z X (us) distance
4 Pillars of the Big Bang theory I. Expansion of the Universe II. Cosmic Microwave Background III. Primordial Nucleosynthesis IV. Evolution of galaxies and large scale structure over the last ~14 billion yrs.
The Big Bang Theory If the Universe was much smaller in the past, it would have been much hotter in the past. Theorists like Robert Dicke in the 1960 s predicted that this glow from the Big Bang could still be observed. The glow from the big bang should be observable as a T=3K Blackbody curve, seen in all directions from empty space.
History of the Universe For the first 300,000 yrs the Universe was so hot that light could not propagate freely. Electrons were not bound to atoms (atoms were ionized) and since light tends to scatter off of free electrons, the Universe was like a thick fog. microwave background light nuclei & free electrons Quarks & primordial particles e e p n n p Big bang e n e p - e - + Observable Universe, Galaxies 300,000 years 3 min 10-20 s 10-35 s
History of the Universe For the first 300,000 yrs the Universe was so hot that light could not propagate freely. Electrons were not bound to atoms (atoms were ionized) and since light tends to scatter off of free electrons, the Universe was like a thick fog. microwave background n e p n n p eno light from this time can reach us e e p - e - + Big bang Observable Universe, Galaxies 300,000 years 3 min 10-20 s 10-35 s
Universe at z > 1000 (t < 300,000 years) e- p e- p e- p e- e- X e- p Location of Milky Way
Universe at z > 1000 (t < 300,000 years) e- p e- p e- p e- e- X e- p Location of Milky Way
Universe at z > 1000 (t < 300,000 years) e- p e- p e- p e- e- X e- p to see back to z >1000, we must be able to see a very distant part of the Universe (relative to us). Location of Milky Way
History of the Universe For the first 300,000 yrs the Universe was so hot that light could not propagate freely. Electrons were not bound to atoms (atoms were ionized) and since light tends to scatter off of free electrons, the Universe was like a thick fog. microwave background n e p n n p eno light from this time can reach us e e p - e - + Big bang Observable Universe, Galaxies 300,000 years 3 min 10-20 s 10-35 s
History of the Universe For the first 300,000 yrs the Universe was so hot that light could not propagate freely. Electrons were not bound to atoms (atoms were ionized) and since light tends to scatter off of free electrons, the Universe was like a thick fog. microwave background n e p n n p eno light from this time can reach us e e p - e - + Big bang Observable Universe, Galaxies 300,000 years 3 min 10-20 s 10-35 s
Universe at z ~ 1000 (t ~ 300,000 years) H H H X H Location of Milky Way
Universe at z ~ 1000 (t ~ 300,000 years) H H H X H Once, e- and protons are able to combine, light is able to travel freely over great distances. Location of Milky Way
Did the Big Bang Happen? At 300,000 years after the Big Bang, the Universe became transparent. At this time the Universe had a temperature of 3000 degrees K. The light from this time should be visible today. It would have redshifted by a factor of 1,000 and look like a T=3K black body.
Did the Big Bang Happen? At 300,000 years after the Big Bang, the Universe became transparent. At this time the Universe had a temperature of 3000 degrees K. The light from this time should be visible today. It would have redshifted by a factor of 1,000 and look like a T=3K black body.
Did the Big Bang Happen? At 300,000 years after the Big Bang, the Universe became transparent. At this time the Universe had a temperature of 3000 degrees K. The light from this time should be visible today. It would have redshifted by a factor of 1,000 and look like a T=3K black body. 1965: Penzias & Wilson discover the glow left over from the Big Bang. Cosmic Microwave Background space is 3 above absolute zero in all directions.
What you see when you take an image of the sky at microwave wavelengths... Is this what you expected?
What you see when you take an image of the sky at microwave wavelengths... Is this what you expected?
What you see when you take an image of the sky at microwave wavelengths... Is this what you expected?
If you remove much of the foreground, you find variations at 1 part in 1000 that look like this... What s going on here?
The CMB as seen by the WMAP satellite. 14
Small, hot universe at 300,000 years After the big bang. T=3,000K Larger but cooler Universe today. T=3K Fig. 19-6, p.394
Small, hot universe at 300,000 years After the big bang. T=3,000K Larger but cooler Universe today. T=3K Fig. 19-6, p.394
Small, hot universe at 300,000 years After the big bang. T=3,000K light from early Universe is redshifted to microwave wavelengths [light originated from z ~ 1000] Larger but cooler Universe today. T=3K Fig. 19-6, p.394
The Cosmological Principle When viewed on sufficiently large scales, the properties of the Universe are the same for all observers....or the Universe looks the same whoever and wherever you are. This implies that the Universe is homogenous and isotropic on large scales (i.e. over large distances). the CMB very much supports this picture.
4 Pillars of the Big Bang theory I. Expansion of the Universe II. Cosmic Microwave Background III. Primordial Nucleosynthesis IV. Evolution of galaxies and large scale structure over the last ~14 billion yrs.
4 Pillars of the Big Bang theory I. Expansion of the Universe II. Cosmic Microwave Background III. Primordial Nucleosynthesis IV. Evolution of galaxies and large scale structure over the last ~14 billion yrs.
Alpher, Bethe, & Gamow (1948) George Gamow predicted background blackbody radiation with T~5K 19
History of the Universe At 300,000 yrs after the big bang, the Universe became transparent to light. This is the era we see from cosmic microwave background radiation. microwave background We can only determine what was happening at this time using theoretical estimates and residual tracers. We cannot see this epoch directly. p n n p n p Big Bang p n - + - 300,000 years 3 min 10-35 s
< 1 sec after the big bang the Universe would have been >1 Billion degrees. At these temperatures, the protons and neutrons are maintained in an equilibrium by the following electronneutrino weak interactions...
< 1 sec after the big bang the Universe would have been >1 Billion degrees. At these temperatures, the protons and neutrons are maintained in an equilibrium by the following electronneutrino weak interactions... Timescale for these interactions depends on the energy of the neutrinos and the number density of particles, such that...
< 1 sec after the big bang the Universe would have been >1 Billion degrees. At these temperatures, the protons and neutrons are maintained in an equilibrium by the following electronneutrino weak interactions... Timescale for these interactions depends on the energy of the neutrinos and the number density of particles, such that... So at T ~ 1 x 10 10 K (or t ~ 0.7s), the timescale exceeds the age of the Universe. What happens then?
So at T ~ 1 x 10 10 K (or t ~ 0.7s), the timescale exceeds the age of the Universe. What happens then?
So at T ~ 1 x 10 10 K (or t ~ 0.7s), the timescale exceeds the age of the Universe. What happens then? Freeze-out!
So at T ~ 1 x 10 10 K (or t ~ 0.7s), the timescale exceeds the age of the Universe. What happens then? Freeze-out! At this point, the number of neutrons to protons will be... [ n ] n + p = 0.21
So at T ~ 1 x 10 10 K (or t ~ 0.7s), the timescale exceeds the age of the Universe. What happens then? Freeze-out! At this point, the number of neutrons to protons will be... [ n ] n + p = 0.21 But neutrons are not stable, so they begin to decay...all the while, the Universe continues to expand and cool.
So neutrons begin to decay, as the Universe continues to expand and cool...at some point, the Universe is cool enough that photons can no longer dissociate light nuclei (i.e. H, He).
So neutrons begin to decay, as the Universe continues to expand and cool...at some point, the Universe is cool enough that photons can no longer dissociate light nuclei (i.e. H, He). And so then protons and neutrons begin to form the light elements at t ~ 300s (or ~5 min later)...
So neutrons begin to decay, as the Universe continues to expand and cool...at some point, the Universe is cool enough that photons can no longer dissociate light nuclei (i.e. H, He). And so then protons and neutrons begin to form the light elements at t ~ 300s (or ~5 min later)... Nearly every neutron that survives ends up in a He nucleus, such that for every 2 neutrons surviving we get 1 4 He.
So, based on our knowledge of neutron decay and the theory of the Big Bang, we would expect that the resulting neutron density at t ~ 300s is... [ n ] n + p = 0.123
So, based on our knowledge of neutron decay and the theory of the Big Bang, we would expect that the resulting neutron density at t ~ 300s is... [ n ] n + p = 0.123 And the corresponding Helium abundance would be... This is a strong prediction from the Big Bang theory. Observations of 4 He, 3 He, D, support the theory.
100 sec after the big bang 100 sec after the big bang the Universe would have been ~1 Billion degrees. This is the relevant temperature and conditions for protons and neutrons to combine to form light nuclei like He. The ratio of Helium to Hydrogen that would be made is predicted very robustly. Observations indicate great agreement with the Primordial Nucleosynthesis predictions. That is, the He/H ratio in old stars agrees with what is expected from the Big Bang.
Why didn t the light elements (i.e. Hydrogen & Helium) form earlier than 3 minutes after the big bang?
Why didn t the light elements (i.e. Hydrogen & Helium) form earlier than 3 minutes after the big bang? It was too hot!
Early Universe Timeline Big Bang t = 0 Nucleosynthesis t ~ 100-300 s Recombination t ~ 400,000 yr sea of elementary particles light elements (e.g. H, He) form sea of electrons and nuclei H and He atoms form; photons able to stream freely (CMB!)
Universe Timeline Big Bang t = 0 Recombination t ~ 400,000 yr Local Universe t = now Nucleosynthesis t ~ 100-300 s H and He atoms form; photons able to stream freely (CMB!) Very little atomic hydrogen observed in empty space.
Microwave Background