Historical Development of Cosmology And models that fit “most” of the data From Geocentric to Dark Matter to Dark Energy to Alternative Gravity on Large Scales Do we really “Know” Anything?

Once Upon a Time We were certain that: The Earth was the Center of the Universe (consistent with most observations) The Earth was motionless (intuitively obvious) All things fell toward the Earth unless they were in the perfect celestial realm

And This is the Known and Absolute Truth Aristotle Places Us in the Center of the Universe And This is the Known and Absolute Truth

Arrogance and the Anomaly Three Choices: 1. Ignore it 2. Fit it to your Bias 3. Throw out your Bias

Three Kinds of Universes The Physical Universe  consists of material objects contained within it The Interacting Universe  material objects communicate/affect each other through long range forces The Geometrical Universe  the space in which the material objects are embedded

Most of what we “know” Comes from observed motions of test particles Within the framework of our current theory of long range forces we determine mass distributions in the Universe 1859 precision observations of the orbit of Mercury DISAGREE with Newtonian Mechanics!  Incomplete Model

The Next Truth Gravity = curvature of space time Total Energy content of the Universe determines its geometry Expansion verified by 1929 1963 MWB accidental discovery verifies Gamow’s fire ball theory  Hot Big Bang

Big Bang Cosmology Is the Universe open or closed? What is the Hubble parameter? What is the age of the Universe? If closed age = 2/3 H-1 1970s Truth: Age of Universe = 17 billion years; Universe is open; H =50 Critical Question: Why is the causally disconnected MWB everywhere the same?

Inflation solves the homogeneity problem with the following consequences Requires the Universe to be “flat” and therefore sets the large scale energy density; W=1 By 1980 it is known that the “material” visible in the Universe provides only W = 0.005 Since we know the “answer” we now require a Dark Matter Dominated Universe And this is now the “Truth”; Wb =0.005; Wdm=.995

The Quest for Dark Matter Well we “know” it must be there  flat rotation curves for galaxies Another Anomaly and a New Problem

Resolution: Galaxies must be surrounded by “Halos” of dark material that contain 90% of the total mass? But wait a minute, this doesn’t actually work since 99.5% needs to be Dark Matter Two types now: Binding Galaxy potentials and distributed background  Particle Physics must be the source

Dark Matter Makes Galaxy Formation Possible  Consistent Idea Purely baryonic fluctuations damp out; Dark Matter is not subject to radiation pressure in the Early Universe DM: several hundred more e-folding times to concentrate itself compared to baryons Expect formed dark matter concentrations which then capture baryonic material  Dark Galaxies?

Now

But (irrelevant) Experimentalists could find no evidence for W =1 ! Observationally, again using the motions of test particles, consistently showed that the ‘dark matter energy density’ was 0.25 of that required to make the Universe flat. Can’t be right since Wdm =1 is the TRUTH

2000: WMAP and the New Truth Where did all the DM go? (It went to the value the observers said it was!) How come there are 8 times more Baryons than there were in 1980? Age of Universe = 13.5 Gyr

So Are we done Yet? Do we now know the certain truth as we once did with Aristotle? If so, the challenge for this century is to determine the physical nature of the Dark Energy just like the (failed) challenge of the last century was to understand the physical nature of the ubiquitous Dark Matter Are there still anomalies within this Truth?

Three Important New Anomalies Remember the “increase” in Baryons as a result of the New Truth? GALEX discovers Stars (lots of them) where there should be none:

Maybe lots of Intergalactic Baryons

PMO: Lots and Lots of Stars

Maybe we still don’t understand Gravity? Maybe gravity works differently on different acceleration scales? (GM/R2) effective scales of 1 A/s2 exist in astrophysical bound systems This means the effective acceleration on an object changes so the force law changes  no need for dark matter (modified dynamics) empirically fits all data as well as the CDM  requires only baryons + dark matter LSB galaxies as natural labs for MOND: .

MOND FITS

Perspectives We have a history of finding “correct” cosmological models We have not yet made the full set of required measurements to properly inventory the Universe Certainty is unwarranted Landau: Cosmologists are usually wrong, but never in doubt