PHYS 420: Astrophysics & Cosmology

Transcription

1 PHYS 420: Astrophysics & Cosmology

2 Dr Richard H. Cyburt Assistant Professor of Physics My office: 402c in the Science Building My phone: (304) My My webpage: In person or is the best way to get a hold of me.

3 My Office Hours TR 5:30-17:00am W 4:00-6:00pm Meetings may also be arranged at other times, by appointment

4 Joint Spring Meeting of the Appalachian and Chesapeake Sections of the AAPT Saturday, April 22 at Concord University ($15/students/guests, $25/AAPT) 8:00 8:45 Registration and Continental Breakfast 8:45 11:45 Contributed Talks and Demos I 11:45 12:45 Lunch 12:45 1:30 Poster Session 1:30 2:30 Invited lecture 2:30 3:15 Contributed Talks and Demos II 3:15 4:15 Invited workshop 4:15 5:00 CSAAPT Business Meeting (all are welcome) Invited lecture: Astrophysical Hoyle State Enhancement, Dr. Richard H. Cyburt, Concord University Invited Workshop: Women and Minorities in the History of Physics, Role Models for Today, Dr. Gregory Good, The Spencer Weart Director, Center for History of Physics American Institute of Physics

5 Douglas Adams Hitchhiker s Guide to the Galaxy

6 Overview of our Galaxy. So far Dark Halo Halo Thin Disk Bulge Thick Disk and Metal-Weak Thick Disk 6

7 chemical evolution All the atoms (except H, He & Li) were created in stars! Zentrum fuer Astronomie und Astrophysik, TU Berlin Pop III: zero-metallicity stars Pop II: old halo stars Pop I: young disk stars We are made of stardust! Þ Old stars contain fewer elements (e.g. iron) than younger stars

8 A long time ago... First stars (100 M ) 2 nd and later generations of stars (<1 M ) Big Bang today first galaxies today s galaxies Larson & Bromm 2001 Cosmic time (not to scale)

9 Newton s Static Universe Universe is static and composed of an infinite number of stars that are scattered randomly throughout an infinite space. Universe is infinitely old and will exist forever without any major changes. Time and Space are steady and independent of one another and any objects in existence within them.

10 Newton s Error If universe is as how Newton describes, then why is the sky dark at night?

11 Olber s Paradox If space goes on forever with stars scattered randomly throughout, then in any line of sight in any direction will eventually run into a star. Using this logic, the sky should be the average brightness of all of these stars; the sky should be as bright as the sun, even at night.

12 But isn t the sky dark at night? Yes, of course - that is what we observe now and have always observed. Something is wrong with Newton s idea of a static, infinite universe.

13 Einstein s Relativity Einstein overturned part of Newton s theory with his theories of special and general relativity - time and space were indeed related, as were the objects existing within them.

14 Special Relativity Time and Space and their rates are intertwined and depend on the motion of the observer (1905). Inertial frames of reference (no acceleration) The speed of light is a law of physics The laws of physics shouldn t change just do to the relative motion of observers.

15 General Relativity Gravity couldn t be made or remade to fit within Special Relativity. Gravitation is a law of physics, but not consistent with Special Relativity??? Gravity bends the fabric of space time - the matter that occupies the universe influences the overall shape of space and the rate of time (1916). Non-inertial frames of reference (acceleration) and you can t tell the difference between accelerating in an elevator and gravity.

16 Implications of Einstein s Ideas Based on the general relativity equations, the structure of universe is either always expanding, always contracting, or always static. To agree with the ideas of the time (Newton s), Einstein added a cosmological constant which yielded a static universe.

17 Cosmological Constant Represents the pressure that allows the universe s expansion to directly balance gravitational collapse due to the objects existing within the universe, thus yielding a static universe. Without this idea of a cosmological constant, Einstein could ve been the first to predict that the universe is not static.

18 Hubble s Discovery Edwin Hubble s observations of remote galaxies, and the redshift of their spectral lines (1924). Hubble noticed that the further away the galaxy, the greater the redshift of its spectral lines. This linear relationship is called Hubble s Law.

19 Redshift The wavelengths of the light emitted by distant objects is elongated as it travels to earth. Longer the light travels, the more it gets redshifted.

20 Hubble s Law v = H 0 d v = recessional velocity of the galaxy H 0 = Hubble constant (rate of expansion today) d = distance of galaxy to earth Galaxies are getting farther apart as time progresses, therefore the universe is expanding.

21 Hubble s Constant Expansion rate measured using Type 1A Supernovae. The age of the universe can be derived from Hubble s constant: T " = $ % = $ & ' $ = ( & ' For example, if H 0 = 73 km/s*mpc, then T 0 = 13.4 Billion years old

22 Age of Universe Currently, after taking into account differences in expansion rate over time and our movement through space: T 0 ~ 13.7 ± 0.2 Gyr Age of stars: ~13.4 Gyr ± 6% Therefore, oldest stars are younger than the age of universe.

23 How the Universe Expands The space between galaxies expands, not the galaxies themselves; objects held together by their own gravity are always contained within a patch of nonexpanding space. Example: raisins in a loaf of bread. As the dough rises, the overall loaf of bread expands; the space between raisins increases but the raisins themselves do not expand.

24 Center of Universe? There is NO CENTER to the universe Expansion looks the same regardless of where you are in the universe. Every point appears to be the center of the expansion, therefore no point is the center. The universe is infinite.

25 Expanding Universe

26 Expanding Universe (Center of Expansion)

27 Expanding Universe (Hubble s Law)

28 Expanding Universe (Center of Expansion)

29 Expanding Universe (Hubble s Law)

30 Center of Universe? There is NO CENTER to the universe Expansion looks the same regardless of where you are in the universe. Every point appears to be the center of the expansion, therefore no point is the center. The universe is infinite.

31 Evidence for Expansion The light from remote galaxies and other objects is redshifted. This redshift is called cosmological redshift because it is caused by the expansion of the universe, not by the actual movement of the object (doppler redshift).

32 Lookback Time The degree of cosmological redshift tells you how far into past you are seeing the object due to the finite speed of light; this value is called Lookback time. However, these values are not always certain because of the expansion of universe was not always constant.

33 Observable Universe Olber s Paradox is solved: due to the finite speed of light, the observable universe does not include the entire universe. Radius of the observable universe depends on the age of the universe and the speed of light: ~47 billion lightyears. Result: Sky is dark at night with points of light (stars, galaxies, etc.) scattered throughout.

34 Origins of the Big Bang Theory Georges Lemaître (1927) expanded on idea of expanding universe, realizing that the universe was smaller yesterday than today, and so on until a day that would not have had a yesterday : the moment of creation. The moment of creation would be the sudden expansion that started the expansion of the universe as we know it today. This idea wasn t widely accepted at first: Fred Hoyle dismissed this hot Big Bang, noting that there wasn t any record or remnants. He argued for a steady state universe.

35 Origins of the Big Bang Theory Alpher & Gamow (1948) suggested that if the universe was created with a hot Big Bang, then: Various elements, such as H and He, would be produced for a few minutes immediately after the Big Bang due to the extremely high temperatures and density of the universe at this time. The high density would cause rapid expansion. As the universe expanded, H and He would cool and condense into stars and galaxies. Today, due to continued cooling, radiation left over from the epoch of recombination, when neutral atoms formed (~380,000 years after Big Bang) should be about 3K. Production of H and He during this time instead of just in H-burning in stars would explain why the H:He ratio of the universe is higher than what could ve been produced by stars alone. Gamow humorously decided to add the name of his friend the eminent physicist Hans Bethe to this paper in order to create the whimsical author list of Alpher, Bethe, Gamow, a play on the Greek letters α, β, and γ (alpha, beta, gamma).

36 Evidence for the Big Bang Theory Gamow s theory was revisted in the 1960 s by Bob Dicke and Jim Peebles of Princeton University. Believed that this cooled radiation would be redshifted to the microwave region of the electromagnetic spectrum. Made a receiver to detect this radiation, but were unsuccessful.

37 Evidence for the Big Bang Theory The radiation, so far undetected by the Princeton team, was posing a problem for NJ Bell Telephone Labs, where Arno Penzias and Robert Wilson were developing a new microwave-satellite technology for phone calls. Puzzled by steady hiss that they received no matter where in the sky they pointed their antenna. This faint background noise they were trying to get rid of was exactly what the Princeton team was trying to detect: evidence of the Big Bang. Signal was not from bird poop!!!!

38 CMB Radiation Detection of this radiation, called Cosmic Microwave Background radiation, won Penzias and Wilson the Nobel Prize for Physics in CMB radiation can be detected by your tv as well - 1% of static seen on a channel that your tv doesn t receive is from the birth of the universe.

39 CMB Radiation Intensity of CMB Radiation reveals origins of universe. However, difficult to detect intensity from Earth- the atmosphere is opaque to wavelengths 10 µm to 1 cm (CMB ~ 1 mm). COBE (Cosmic Background Explorer) 1989: detector outside the atmosphere: Measured the blackbody spectrum of CMB radiation to be at T = K - consistent with theory. CMB radiation almost entirely isotropic; CMB is slightly warmer in direction of Leo and slightly cooler in direction of Aquarius. WMAP (Wilkinson Microwave Anisotropy Probe) (2002) improved picture of CMB Radiation. PLANCK Satellite an even better picture

40 CMB Radiation Intensity of CMB Radiation reveals origins of universe. However, difficult to detect intensity from Earth- the atmosphere is opaque to wavelengths 10 µm to 1 cm (CMB ~ 1 mm). COBE (Cosmic Background Explorer) 1989: detector outside the atmosphere: Measured the blackbody spectrum of CMB radiation to be at T = K - consistent with theory. CMB radiation almost entirely isotropic; CMB is slightly warmer in direction of Leo and slightly cooler in direction of Aquarius. WMAP (Wilkinson Microwave Anisotropy Probe) (2002) improved picture of CMB Radiation. PLANCK Satellite an even better picture

41 Historical Observations of the CMB and Anisotropy

42 CMB Radiation Radiation appears to be mostly smooth, but there are slight variations in temperature that show that matter had started to clump in the early universe clumps of matter formed the galaxies and stars see today. Sound waves in early universe are recorded in this radiation; by studying the characteristics of these sound waves, we can find out about the conditions of the early universe.

43 CMB Radiation Radiation appears to be mostly smooth, but there are slight variations in temperature that show that matter had started to clump in the early universe - clumps of matter formed the galaxies and stars see today. Sound waves in early universe are recorded in this radiation; by studying the characteristics of these sound waves, we can find out about the conditions of the early universe.

44 Horizon Problem Despite all of the success with the Big Bang Theory so far, the horizon problem was still yet to be solved. The temperature of the CMB radiation was the ~same no matter where you look in the sky, indicating that some how information linking all parts of the sky was traveling faster than the speed of light. Also, information from one side of the sky at 100,000 years old (horizon is 100,000 light years in diameter) differed from the other side of the sky by 10 million light years times the diameter of the horizon. How is this possible?

45 Inflation Theory Alan Guth (1970s) had a solution: The universe must have expanded exponentially very early for a short period of time. This would account for the clumping of matter.

46 Evidence for Inflation Theory Guth predicted that the average density of the universe should be equal to the critical density (6 protons/m 3 ) This was confirmed by powerful telescopes. Evidence from CMB shows that the clumping of matter is consistent with the amount of accelerated expansion during inflation.

47 Extent of Inflation Today, evidence and theory show that: At T = sec, universe d = cm Between T = sec and T = sec, the universe expanded exponentially by a factor of For the briefest moment, the universe expanded faster than the speed of light.

48 Big Bang Theory: Timeline of Universe Hubble s Law shows that the universe has been expanding for billions of years - the universe is denser the further back in time you look. At some point, you reach an infinitely dense point at which T age of universe = 0 Big Bang

49 T = 0 seconds to seconds BIG BANG occurs. Something causes infinitely dense point to expand (into Nothing). Density of universe is so high that time and space are curled up and the laws of physics that we know today do not apply. All four forces in nature were unified. This is time is called the Planck Time.

50 Separation of Forces After the Planck time, the temperature had decreased K and gravity was the first force to separate. The remaining three forces were still united - these are the conditions that particle physicists today try to replicate.

51 T = to seconds Inflation caused the size to the universe to increase exponentially by a factor of This time is called the inflationary epoch.

52 After Inflation Stops Matter is created: Interactions between the Inflation field and elementary particles acts like a friction to stop the accelerated expansion and the heat of friction produce elementary particles such as photons, electrons and positrons, and quarks and antiquarks. Pair production continues until one of particle could no longer be produced - pair annihilation happens - result: symmetry breaking. Reason for slight excess of matter over antimatter is because of an unknown reaction known as baryogenesis, in which conservation of baryon number is violated. Pair Production occurred until T = 6GK, but pair annihilation happens independent of temperature.

53 Particle Production in Early Universe As the size of the universe increases and the temperature decreases, the particles produced are of decreasing energy. The fundamental forces and parameters of elementary particles at the time that symmetry was broken are the same as they are today. The time between the birth of the universe and t = seconds is rather unknown, but we can speculate what is happening based on other observations; beyond this time is less speculative as these are conditions that particle physicist try to replicate.

54 T = 10-6 seconds Temperature has cooled enough for baryons (Protons, Neutrons) and mesons (pions) to form. The strong force wins over quarks/antiquarks ability to bounce freely. Once the temperature has decreased enough and hadrons form quark/antiquark pairs are no longer made. Also, at this temperature, most particles are no longer moving relativistically, so the universe becomes dominated by the higher energy photons (radiation-dominated universe).

55 T = few minutes Temperature ~ 1 GK, density ~ that of air. Neutrons combine with protons making deuterium and helium nuclei, and some protons remain independent (hydrogen nuclei). Called Big Bang nucleosynthesis. Temperature is still too high to form atoms as they would be ionized immediately. The universe would appear opaque during all this time because photons and matter would be interacting due to high temperatures.

56 T = 379,000 years Universe is now cool enough that matter energy is greater than radiative energy, thus allowing atoms to form. Radiation is decoupled from matter and photons are free-streamed throughout space - origin of CMB radiation. This time is known as the epoch of recombination. Universe is now matter-dominated.

57 T ~ 400 million years Since epoch of recombination, slightly denser regions attracted matter nearby and the first stars begin to form. Regions continue to acquire matter and other objects like galaxies and gas clouds form. Universe begins to look like how we know it today (still expanding and still cooling).

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59 Matter in the Universe Today Evidence gathered from CMB shows that all of the matter in the universe is composed of three types of matter: Cold dark matter (don t know what this is) Hot dark matter (neutrinos) Baryonic matter (stuff made from neutrons & protons) Cold dark matter accounts for ~82% of all matter and hot dark matter and baryonic matter combined account for the remaining ~18%.

60 Nature of Expansion Today Evidence of Type 1a supernovae and CMB radiation show that the expansion is accelerating, driven by dark energy. Dark energy comprises ~72% of all energy and permeates all space. It is likely that this dark energy has always been throughout the universe, but when the universe was younger and much smaller, gravity was stronger than dark energy. This acceleration could be described by Einstein s cosmological constant. Today, dark energy is still very misunderstood.

61 Expansion & Fate of Universe

62 Fate of the Universe

63 Research Today Today, particle accelerators such as the LHC are trying to replicate conditions just after the Big Bang so that we understand how the universe formed. Currently, all cosmic evolution after inflationary epoch can be modeled and described pretty accurately, but the time before this (10-15 sec) is basically unknown; understanding this time remains one of the greatest mysteries in physics.

64 Remaining Questions What is dark matter? What is dark energy? Can dark energy and matter be detected and studied in labs? What happened from the birth of the universe, at the instance of the Big Bang, until the end of the inflationary epoch? What caused the Big Bang? What is the ultimate fate of the universe?