The Observers Universe

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The Theorists Universe

Cosmological Issues:

• Particle Generation and Their Masses
• Baryon Content of the Universe
• Matter (light + dark) Density in the Universe
• Nature of the Dark Matter
• Formation of Galaxies
• Large Scale Structure
• Determination of Cosmological Parameters

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Particle Generation and Their Masses:

Masses of quarks endowed during some phase transistion? --> sets the entropy of the Universe is well?

SU(10), SU(16), SU(??) --> how will we ever know which one is correct?

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Baryon Content of the Universe:

Places where Baryons can reside:
• Easy to detect Galaxies
• Hard to detect Galaxies
• Cosmological Backgrounds --> QSO absorption lines

Constraints on Baryon Density:

• Abudances of helium, lithium --> 0.04 -- 0.07 (for h =1)
• Number density of galaxies
• Observations of Deuterium --> if believed then baryon density is around 0.02

Conclusion --> We don't yet have an accurate census of the repositories of baryonic matter in the Universe

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Matter (light + dark) Density in the Universe:
• Upper limit on Baryon density is likely to be 0.07.
• Galaxies (all types) themselves contribute 0.04 at most
• Inflation demands Omega = 1 --> Observationally, most evidence points to Omega = 0.2
  Ways to Measure Omega:
  • Determine deceleration parameter --> SN Ia
  • Measure deviations from Hubble Flow due to infall to overdense regions
  • Measure the Power Spectrum of Large Scale Structure

Conclusions: Dynamical determinations of Omega are quite difficult but over most scales they indicate a value less than 0.2

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Nature of the Dark Matter:
How do we know its there?
• Inflation says so
• Galaxy rotation curves --> 0.1
• Motions in Virialized Clusters --> 0.1
• Fluctuations in DM required for structure formation

What is it?
• Baryonic Candidates --> stellar remanats, brown dwarfs --> fine as long as the Universe is open
• If Omega = 1 --> its a particle

Detecting Dark Matter:

• MACHO Lensing survey
• MACHO is hard and may not produce unambiguous results
• SSC --> dead
• Solar Neutrino Experiment --> suggest small mass for neutrino? --> note: for h=1 need neutrino mass of around 30 eV for closure
• Other Accelerator Experiments

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Formation of Galaxies
This is understood in principle

In practice, there are lots of difficulties:

• Unknown initial conditions
• Unknown role and nature of dark matter
• Unknown primoridal fluctuation spectrum although there are now constraints from COBE
• Not predicted in HDM scenarios
• Too many galaxies form in CDM scenarios
• Galaxy formation has never been observed!

• Oh hell let's just simulate it

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Large Scale Structure:
• Determined from Redshift Surveys
• Extant surveys indicate three complicating themes:
  • We haven't surveyed a fair volume of the Universe yet
  • There is power on very large scales
  • There are suggestions of motions on very large scales which violates COBE anisotropy constraints --> galaxies are not the same at the 10% level!

  When redshift surveys are combined with the COBE constraint one can make the following definitive statement:

  Standard, biased CDM can't account simultaneously for both the small scale and large scale structure. Unbiased CDM does not help much either.

  Now one is driven to consider:

• Mixed dark matter
• Tilted Inflationary scenarios (non-Gaussian distributions)
• Non-zero Lambda

  So, things are getting complicated and there are not yet good enough observations to distinguish between the above combination of models (meaning that theory can now fill the literature ).

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Determination of Cosmological Parameters:

Hubble Space Telescope Key Project --> determination of h

First results (Oct. 94): h = 0.8 +/- 0.17

When corrected for small systematic errors h = 0.9 +/- 0.17

But hey there are now stars that are older than the Universe!

GO WHERE THE DATA LEADS --> Lambda is not ZERO!

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