A BRIEF INTRODUCTION TO BIG BANG COSMOLOGY SZYDAGIS02.16.2015 Top image: Big Bang art via NASA/JPL Photo of Alan Guth ©2015 CTMG - A Chicago Tribune website.

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Top image: Big Bang art via NASA/JPL

Photo of Alan Guth ©2015 CTMG - A Chicago Tribune website

A COSM0CHEMISTRY MYSTERYWhere did all the (92) naturally

occurring elements come from? Proportions=?

We have known for a very long time that the universe is ~75% hydrogen and ~25% helium overall; the heaviest elements, which comprise any rocky planets like the earth ~1-2%. Why??

Couldn’t even attempt to answer until we learned of the atomic structure of matter (protons, neutrons, electrons), radioactivity, nuclear fission and fusion

Could “stellar furnaces,” the nuclear “ovens” at the cores of stars be the answer? Better yet, what about the primeval fireball from the Big Bang?

"Periodic Table overview (standard)" by DePiep - Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons

By the early 20th century, we learned that our hot sun is composed mostly of hydrogen (element #1, the lightest, all the way at the upper left) and that it produces its energy via nuclear fusion (hydrogen nuclei, 1 proton each, bang together to form helium nuclei, defined as 2 protons, & n’s)How did the universe start out? Various ideas put forth

- A gas of hydrogen, collapsing first before expanding- Gamow’s ylem, a hot gas of free neutrons that decayedinto protons and electrons within minutes, but not entirely:some neutrons survive to bind with protons into nuclei2

BIG BANG NUCLEOSYNTHESIS (BBN)The Big Bang theory gets the big picture correct right off the bat (three-quarters of

cosmos hydrogen, spread evenly throughout space) but the devil is in the details:

Big Bang universe not hot enough to manufacture the heavier elements, so the steady state theory advocates were partially right: you need the stars (too). But, fatal flaw (one of many) of steady state theory was that it could not explain the origin and distribution of the lightest chemical elements, the opposite problem

A one-two punch: first you get hydrogen and helium and a few other light nuclei in the beginning, heavy elements much later in stars, which turn hydrogen into helium (with processes akin to those within H-bomb), then helium into lithium & so forth..

* Protons and electrons main stable subatomic particles. Protons formed from quarks as universe cooled, then protons+electrons formed hydrogen atoms

* Not end of story: where is the helium? Well, at extreme temperatures (early universe) protons and electrons can react to make neutrons, then protons and neutrons combine into deuterium nuclei, two of which make helium (2p, 2n)

* Final trick: some unbound neutrons decayed (10-min. half-life). ~80% survive

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Rejected explanations for observed hydrogen/helium ratio- All the stars in the universe going for all of the

billions of years of its existence would account for only ~1% of the universe being helium, not a quarter, and be hidden

- Universe “created” with amount of helium it has today

GALAXY SPREAD OVER SPACE-TIMENucleosynthesis was the first of many problems with the

steady state theory. Next came the measureable distribution of matter (galaxies) across time and space

* Steady state expectation: Wide variation in galactic ages, and no significant change in density looking far away through space (that is, looking back in time). Eternal universe with matter being created leads (very slowly over time) to new galaxies coalescing, replacing old ones lost into deep space due to expansion

* Big Bang prediction: Galactic density reduction expected when looking further into the past (all matter created in beginning, so as universe expands separation increases), with galaxy formation eventually seen, then (far) no galaxies at all

Radio astronomer Martin Ryle studied “radio galaxies,” surprisingly intense sources of radio waves in the sky, discovering that they were dispersed UNEVENLY: larger numbers in distant past (greatest distance from us), plus more densely packed

* Results initially controversial, but repeatedly confirmed: present state of cosmos much different, on large scales, than conditions billions of years ago 4

NASA

Quasars: the evidence in favor of the Big Bang mounts- Extremely high redshifts, so ancient, close to

beginning- Nothing like anything in nearby young cosmos:

primordial- Very high levels of radiation from very small

areas- Now understood to be active cores of baby

galaxies

THREE NEW COSMOLOGICAL PARADOXES The last two very major points plus famous Cosmic Microwave Background (CMB)

radiation we already discussed in brief seemed to seal the deal on the Big Bang, but that doesn’t mean that there weren’t difficulties critics could seize upon

Flatness: There are an infinite number of possible positively- and negatively-curved geometries, but all evidence (thus improbably) points to the special case of “flatness” (not 2D; we mean the 3D analog of flat) for our universe (parallel lines stay parallel, angles of triangles add up to 180°, density, best fit to CMB data)

Isotropy of the early universe: One can understand why the CMB should come from all directions, but not necessarily why it should be so homogeneous (uniform, identical in intensity): nearly the very same temperature across all parts of the sky (to much better than 1%), disparate parts of universe unable to “communicate”

* Shouldn’t be so, as information cannot travel faster than the speed of light, even if universe can expand faster than it, so regions too far apart should appear distinct, but they look the same. Impossible to establish/maintain w/o contact?

* Other properties: there are exceptions (bubbles, voids), but on the largest scales there are quite similar numbers of galaxies in every direction, and so on

Exotic relics missing: Magnetic monopoles (HEAVY), cosmic strings, cosmic rotation

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A list of old, some failed, explanations for above enigmas- Universe flat within the error bars of our

observations, but could be quite curved: new CMB data killed this idea

- Universe isotropic (smooth) because started out smooth: intellectually unsatisfying (special initial conditions)

- All Theories of Everything wrong: GUT, superstring, etc.

NASA, ESA, S. Beckwith (STScI) and HUDF Team.

HOW CAN THE UNIVERSE BE CALLED ‘FLAT’?

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Image credit: Ned Wright's cosmology tutorial.

More than just statistical consistency, or just an approximation

We have the extreme precision now with CMB measurements to claim that if the universe possesses a curved geometry, its radius of curvature is more than 2 orders of magnitude (x100) larger than the visible universe (defined by light)

* It’s all about scales: on small scales even the earth appears flatThis and galactic homogeneity

not a problem for an eternal universe, but CMB is nail in coffin: why same in every location (and coming from all directions)? Less of problem for Bang

Remember: universe definitely not a literal sphere. Galaxies NOT “inside”

(Undetectable curvature)

INFLATION, A SOLUTION OF EPIC PROPORTIONSBrief period of extremely fast (exponentially so) expansion immediately after the

start, expanding far faster than light initially (fine by SR) which is key. Alan Guth’s idea

* Universe growing from Planck scale, smallest length it makes any sense to talk about (10-35 m, domain of superstrings and quantum fluctuations) to ~O(1-10) m

Neatly solves ALL problems (but backwards: not derivable from first principles – yet!)

* Naturally produces a high degree of flatness, with a uniform rate of expansion

* Areas apparently causally disconnected were actually close together early on

* Predicted monopoles gone, spread out evenly (~1!) across vast reaches of space

* Bonus: Einstein’s intrinsic vacuum energy (the cosmological constant L) was not entirely crazy (but NOT constant*). Resurrected for inflationary theory, to provide negative pressure (not density) field rapidly pushing space out (repulsion)

Too good to be true? Maybe; a catch: What caused explosion in first place? Ad hoc

* Hope is a full theory of quantum gravity provides potential explanation one day, demonstrating that had to be so because of intrinsic properties of quantum foam

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The mysteries of the vacuum energy (intrinsic to space-time)- A repulsive force that can effectively

counteract gravity- Akin (pressure<0) to Hoyle infamous C-field

too: constant creation. New space AND new particles (from energy)

- Though Einstein scrapped it (equal to 0) eventually,

later we realized has to exist because vacuum isn’t empty

* Falls to 0 as expansion slows (Or does it?)

Think continuously compounded interest!

EXPLOSIVE EXPLANATIONS & CONSEQUENCESInflation: no highly specialized initial conditions needed for a smooth cosmos,

and no contact necessitated amongst the far-flung portions of deep space

* Energy density of the vacuum dissipates into particles and radiation we know and love today. These make + feel gravity, and accelerating expansion coasts

* A phase transition occurred, with beautiful (force) symmetry of universe broken, from accelerating (increasing) expansion rate to decelerating one (decreasing)

https://www.youtube.com/watch?v=xFRu2mt6SgQ (glass of water = universe!)

* BUT, could have been delayed: the universe may have supercooled, akin to water being cooled below the freezing point without freezing. This would have led to an energy-packed unstable equilibrium with a L-like anti-gravitational effect

https://www.youtube.com/watch?v=DpiUZI_3o8s (same as the above)

Natural explanation of cosmic mass-energy density being near ‘critical’ (flatness)

Total energy of entire universe at any time = zero? Still need some initial conditions…

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So we DO have a start for an explanation for inflation- Pent-up energy from before symmetry breaks

adds to *already-existing* expansion (Big Bang) and that’s why leads to an acceleration in it. (But what stops this?)

- Eventually spontaneous cessation, even without “disturbance,” once universe over-cooled enough

CONCLUSIONS AND HOMEWORK ASSIGNMENTSLast slide is not the whole story: wrong (the math just doesn’t work out)

despite making for an elegant picture, so we don’t use fast phase transition analogy today

* Replaced with slow transition, then with chaotic inflation (quantum mechanics)

* It is true that “empty” space is teeming with countless tiny subatomic particles, zipping & popping in/out of existence (we shall revisit the concept again, again)

* Critic: “convenient” how at least one type of particle, the ultra-heavy monopole, forever beyond our reach (testable theory? Is OK if other predictions observable)

* Zero-energy argument is slippery, because potential especially ill-defined

Next time we shall learn the tale of the history and evolution of the universe, from the Big Bang, through inflation, and ending with contemporary times. You will also hear about the latest estimates of both its current (visible and total) size, and age

Links to today’s reading (multiple) on course site next to link for this powerpoint

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-Spanish proverb

TIME PERMITTING: QUESTION FOR DISCUSSIONUsing the tools and ideas you already know and/or you are all

already familiar with, either from taking this course, or from external sources, how could you estimate the present-day ‘age’ of our Universe?

* It is NOT necessary to tell me any numbers. Just discuss your methodology.

* We will not discuss the correct answer until next class. Time for introspection

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