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Why are we here?. Dr Martin Hendry University of Glasgow. Why are we here?…. The period of inflation in the very early Universe was invoked to explain some apparent ‘fine tuning’ problems. If the Universe is now inflating, this presents a new set of ‘fine tuning’ problems. Atoms.

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## Dr Martin Hendry University of Glasgow

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**Why are**we here? Dr Martin Hendry University of Glasgow**Why are we here?….**The period of inflation in the very early Universe was invoked to explain some apparent ‘fine tuning’ problems. If the Universe is now inflating, this presents a new set of ‘fine tuning’ problems**Atoms**Cold Dark Matter Dark Energy State of the Universe – Nov 2003**Atoms**Cold Dark Matter Dark Energy State of the Universe – Nov 2003 Why does 96% of the Universe consist of ‘strange’ matter and energy?**General Relativity:-**Geometry matter / energy “Spacetime tells matter how to move and matter tells spacetime how to curve” Einstein’s Field Equations Ricci tensor Metric tensor Einstein tensor Energy-momentum tensor of gravitating mass-energy Curvature scalar**General Relativity:-**Geometry matter / energy “Spacetime tells matter how to move and matter tells spacetime how to curve” Einstein’s Field Equations Treating the Universe as a perfect fluid, can solve equations to determine the pressure and density, and how they evolve**Einstein originally sought static solution but this isn’t**possible, for ‘normal’ pressure and density He added a ‘cosmological constant’ to the field equations Can tune to give static Universe, but unstable (and Hubble expansion made idea redundant anyway!)**But what is ?…**Particle physics motivates as energy density of the vacuum but scaling arguments suggest:- So historically it was easier to believe**Re-expressing Friedmann’s Equations**At any time Dimensionless matter density Dimensionless curvature density Dimensionless vacuum energy density**Re-expressing Friedmann’s Equations**At any time If the Universe is flat then Dimensionless matter density Dimensionless curvature density Dimensionless vacuum energy density**Atoms**Cold Dark Matter Dark Energy State of the Universe – Nov 2003**Value of**Present-day If the Concordance Model is right, we live at a special epoch. Why?…**Hydrogen fusion – fuelling a star’s nuclear furnace**E = mc 2**This has led to more general Dark Energy or Quintessence**models: Evolving scalar field which ‘tracks’ the matter density Convenient parametrisation: ‘Equation of State’ Can we measure w(z) ? Pressure Density**SNIa at z = 0.5**At low redshift, SN1a essentially measure the deceleration parameter Adapted from Schmidt (2002)**SNIa at z = 1.0**At low redshift, SN1a essentially measure the deceleration parameter Adapted from Schmidt (2002)**SNIa at 0.5<z<1.0**At low redshift, SN1a essentially measure the deceleration parameter Adapted from Schmidt (2002)**SNIa measure:-**CMBR measures:- Together, can constrain:- Tegmark et al (1998)**Can we distinguish a constant L term from quintessence?…**Not from current ground-based SN observations (combined with e.g. LSS) Adapted from Schmidt (2002)**Can we distinguish a constant L term from quintessence?…**Not from current ground-based SN observations (combined with e.g. LSS)… …or from future ground-based observations (even with LSS + CMBR) Adapted from Schmidt (2002)**Can we distinguish a constant L term from quintessence?…**Not from current ground-based SN observations (combined with e.g. LSS)… …or from future ground-based observations (even with LSS + CMBR) Adapted from Schmidt (2002)**Can we distinguish a constant L term from quintessence?…**Not from current ground-based SN observations (combined with e.g. LSS)… …or from future ground-based observations (even with LSS + CMBR) Main goal of the SNAP satellite (launch ~2010?) Adapted from Schmidt (2002)

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