How Can We Know What Happened almost 14 Billion Years Ago

Transcription

1 How Can We Know What Happened almost 14 Billion Years Ago May 30th, 2014 Amber Miller Columbia University

2 Evidence: ev-i-dence noun : Something which shows that something else exists or is true --- Webster

3 Reports that say there s that something hasn t happened are always interesting to me, because as we know, there are known knowns; there are things that we know that we know. We also know that there are know unknowns; that is to say we know there are some things that we do not know. But there are also unknown unknowns, the ones we don t know we don t know. --- Rumsfeld

4 In Modern Physics, it really is possible to define knowns, known unknowns and unknown unknowns precisely and clearly. but. A mess is made of this in communicating results to the public that generates very unfortunate misunderstandings.

5 Consequently, we spend much time in the popular press speculating that a new discovery will render what is understood to that point wrong and therefore that we cannot trust what we think we know.

6 Consequently, we spend much time in the popular press speculating that a new discovery will render what is understood to that point wrong and therefore that we cannot trust what we think we know. So, where do we go wrong? 1) Poor Communication 1) Scientists do a poor job of explaining clearly what Is known and what is conjecture 2) Journalists and the public do a poor job of understanding the distinction and therefore looking to draw it out of the scientist 3) Everyone gets too drawn into the sensationalism of explaining how bizarre strange things are, and how much out there we have no idea about. Also - how exciting some new discovery is, and over-playing the scope of how much has changed. 2) System Complexity

7 Complexity Economics Cosmology A (surprisingly) simple system that works well as an illustration Biology Physics/Cosmology Chemistry

8 Paradigm Shift in Natural Philosophy The Copernican Revolution Ptolemaic View of the Universe: The Earth is at the Center, and all the heavenly bodies orbit the Earth. Copernican View of the Universe: The Earth and the other planets revolve around the sun Examples of Objections to the Copernican view (16 th century and before): 1) Theocratic: The Universe was created for mankind, so why was the Earth not at the center of the Universe? 2) Aristotelian: Why would the Earth and the Heavenly bodies behave in such a similar fashion if the Earth is composed of the four elements and the heavenly bodies are composed of quintessence Note: 1) No established protocol for experimentation with the goal of verification, falsification, or discrimination Between competing ideas 2) Paradigm shift from one to the other characterized by demonstration that previous paradigm provided an incorrect description of the physical system

9 Paradigm Shift in Natural Philosophy The Copernican Revolution The Copernican revolution wasn t complete until the work by Sir Isaac Newton: 1) Put into place a simple, quantitative, mathematical, physical model that served to explain a wide range of observed phenomena 2) Outlined a framework for the investigation of physical phenomena that forms the basis of the modern scientific method Image credit: britannica

10 We do teach this kind of.. Steps in the Scientific Method: 1) Observe a natural phenomenon 2) Form an idea as to why the observed phenomenon takes place 3) Elevate the idea to a theory by formulating a quantitative description of the behavior of the phenomenon. 4) Formulate quantitative experiments that can be used to test the theory and evaluate the range of conditions under which it is valid. For an theory to be elevated to law of nature, it must explain the observed phenomenon and predict behavior not yet seen 5) Identify a law of nature through careful experimental verification of a theory Law of Nature (practical definition) - a set of underlying principles governing the quantitative behavior of an observable phenomenon, valid under the range of conditions that have been experimentally verified in a repeatable fashion

11 The KEY message to the non-scientist that is so often lost: Once a theory has been proven correct, one does not ever expect to find that the it is an invalid description under the range of conditions for which it has been experimentally verified However One does expect that given a set of conditions not yet explored, one may find interesting phenomena that do not conform to the law under test. AND An entirely new way of conceptualizing the underlying drivers behind a phenomenon may be discovered.

12 Paradigm Shift in Modern Physics Newton s Universal Law of Gravitation à Einstein s Law of General Relativity F 12 F 21 Newton Einstein

13 Isn t the Big Bang just a theory?

14 We observe that the universe is expanding (galaxies are traveling more quickly the further away they are from us in all directions) If you naively run the clock backward and imagine what would happen if one were to compress all the matter and energy in the universe to smaller and smaller size, you expect it to get hotter and hotter. If the universe gets to temperatures of many thousands of degrees, you expect it to be in the form of a plasma (like the inside of a star) Big Bang Theory: At some point in the past, the universe must have be infinitely hot and dense and has expanded and cooled to the present form

15 And stars, galaxies, etc.? Where did they come from? a) IF there were fluctuations in the density of the plasma.. b) Then, when the universe cools enough for the formation of neutral atoms, matter will begin to condense as gravity pulls matter from regions of lower density onto regions of higher density c) When the matter in the dense regions is sufficiently compressed, it will heat up to temperatures high enough to ignite nuclear fusion, the engine of stars, generating the formation of the first stars (all natural consequence of wellestablished and basic physics) d) Over time, more stars will turn on. Galaxy chains will form along naturally occurring filamentary structure e) Eventually billions upon billions of stars and galaxies will form

16 Then the universe started as a hot, dense plasma everywhere in the universe So, If the Big Bang Model is Correct 1) Radiation from the plasma should be observable today in all directions on the sky like a baby picture of the universe 2) Plasmas emit in a very simple way, so the radiation should have a well-predicted and quantitatively measurable character (Called a blackbody) 3) There should be tiny density fluctuations (viewed as inhomogeneities in the observed blackbody temperature) with precisely the same statistical characteristics as required to produce the structure we see in the universe today Note: No other reason to expect radiation of this kind out in the universe

17 If the Big Bang Model is Correct This radiation was first predicted by George Gamow in 1948, and by Ralph Alpher and Robert Herman in 1950 In 1965, Dicke, Peebles, Roll and Wilkinson at Princeton University were building an experiment to search for this microwave light from the early universe.

18 Meanwhile Researchers at Bell Laboratories working on a communicatons antenna had encountered noise on their system that they could not explain. Frustrated, they tried everything to get rid of it. The leading theory as to what was causing this pesky signal was pigeon excrement on the antenna 1965

19 The Nobel Prize in Physics 1978 Press Release 17 October 1978 The Royal Swedish Academy of Sciences has decided to award the Nobel Prize in Physics for 2006 jointly to Arno Penzias Bell Laboratories Holmdel, NJ, USA 1965 and Robert Wilson Bell Laboratories Holmdel, NJ, USA "for their discovery of cosmic microwave background radiation".

20 The COBE Satellite The uniform CMB (one temperature to one part in 100,000) The motion of the Earth respect to the rest frame of the CMB (measures the combination of our motion around the sun, the motion of the sun around the galaxy, and the motion of our galaxy relative to local galaxies Does NOT imply special place in the universe The first map of the primordial fluctuations that seeded the structure in the universe

21 Measuring the CMB spectrum ± K The COBE Satellite The Hot Early Universe was a very good (ideal actually) blackbody emitter è irrefutable support for hot big bang model!

22 The Nobel Prize in Physics 2006 Press Release 3 October 2006 The Royal Swedish Academy of Sciences has decided to award the Nobel Prize in Physics for 2006 jointly to John C. Mather NASA Goddard Space Flight Center, Greenbelt, MD, USA, and George F. Smoot University of California, Berkeley, CA, USA "for their discovery of the blackbody form and anisotropy of the cosmic microwave background radiation".

23 WMAP Map of primordial density fluctuations (NASA)

24 A simple example of what can the character of these fluctuations tell us WMAP Team:

25 Isn t the Big Bang just a theory? NO

26 We know that our universe started hot and dense We know how we got from initial density fluctuations to the complex structure in our modern universe. But, there are open questions very interesting ones Where did the hot dense universe come from? Where did the density fluctuations come from? And, there are some perplexing things

27 The Flatness Problem Ω =(gravitational potential energy/kinetic energy of expansion) There are three possible cases: 1) Ω=1 normal euclidian geometry - no spacetime curvature 2) Ω>1 spacetime is positively curved 3) Ω<1 spacetime is negatively curved Ω can take on any number but only certain values of Ω result in a habitable universe If Ω < galaxies can't form, IF Ω > 5 the Universe is younger than the Earth

28 The Flatness Problem Ø The measured value is Ω = ± (WMAP + BAO + SN) Ø Strange that it would just happen to be Ω =1 Ø Even stranger because Ω =1 is an unstable point. Ø Ω slightly above or below 1 in the early Universe rapidly grows (under normal expansion conditions) to much less than 1 or much larger than 1 as time passes. Ø After several billion years, Ω would have grown or shrunk, to present-day values of Ω >>1 or Ω <<1.

29 The Horizon Problem Figure: Whittle's Teaching Company Course

30 Inflation Universe << 1 sec old q Our four dimensional world is created from SOMETHING (very active area of research - string theory, M-theory, etc.) We have ideas but this is still wide open q The universe starts out microscopic in size with tiny quantum fluctuations q The entire universe undergoes a super-rapid expansion (expansion is completed by the time the universe is ~10-35 seconds old) to macroscopic (but still small ~1m in diameter) size q The Quantum fluctuations are stretched with the rest of the universe to cosmic scales, seeding the density fluctuations that lead to structure in the universe

31 Inflation solves the horizon problem An inflationary (superluminal) expansion in the early universe means that pieces of the current Universe that are not in causal contact today could have been in causal contact at earlier times

32 Inflation solves the flatness problem Inflation expands the size of the universe by or The universe before inflation can have an arbitrary amount of curvature Inflation expands the universe by such a large amount that our entire current universe is very tiny part of the original universe expanded by a HUGE amount è curvature is exponentially suppressed leading naturally to a flat universe with no fine tuning!

33 Inflation is the leading theory that provides a compelling model of the earliest moments in the history of the universe and is consistent with the data But IS still just a theory Just like the Big Bang, Inflation Predicts something that had never been observed: Theories of inflation predict that gravity waves are generated during the inflationary epoch (when the universe was much MUCH less than one second old) When the CMB photons are emitted, these gravitational waves, traveling through space, should leave a tiny imprint on the CMB

34 The gravity waves should polarize the light a little bit in a very particular way Looking at the same CMB light that we have been measuring all along, we look for this tiny polarization the most direct way we have to observe the universe when it was much less than one second old From Seljak and Zaldarriaga

35 Ripped from Recent Headlines

36 Then again

37 Concluding Paragraph BICEP2 paper

38 BICEP2 Region From: Liu, Mertsch, and Sarkar, FINGERPRINTS OF GALACTIC LOOP I ON THE COSMIC MICROWAVE BACKGROUND WMAP9 K-band polarised intensity map

39 Known Knowns (Will never change) Known Unknowns (Actively Seeking Answers) Unknown Unknowns (Surprises) Our 3+1 dimensional universe went through an early hot, dense phase It has been expanding an cooling since then Is Inflation the right paradigm to explain the origin of these fluctuations? If so, which specific inflationary model is correct (there are many)? Fluctuations in the density of the primordial plasma seeded the formation of structure If not, which alternative model (if any) best fits the data???? geometry, age, composition, broad picture of structure formation history, expansion rate, etc. Will this all contribute to the transition to a new paradigm in which the physics of gravity and quantum mechanics are unified????