One of the few ways we can learn about the composition, age and fate of the Universe is by measuring its expansion rate and its expansion history. Assuming that the Universe is homogeneous and isotropic on large scales and that it obeys General Relativity, we can derive an equation of motion for the Universe, the Friedmann equation, which describes how the scale factor of the Universe, a(t) changes due to the physics and composition of the Universe. Most of what we know about the Universe from this route comes from measuring just the first two derivatives of a(t), parameters known as the Hubble constant (a dot) and the deceleration parameter (a double-dot). These quantities can be measured by estimating distances to exploding and pulsating stars. In 1998, two teams (including the High-z Team and this author) used measurements of supernovae to show that the deceleration parameter was negative, that is, the expansion is now accelerating due to mysterious dark energy. Understanding the nature of dark energy remains one of the biggest goals of modern physics. More recently, the author has been working to improve measurements of the other parameter, the Hubble constant, which measures the present expansion rate. The Hubble constant remains one of the most important parameters in the cosmological model, setting the size and age scales of the Universe. Present uncertainties in the cosmological model including the nature of dark energy, the properties of neutrinos and the scale of departures from flat geometry can be constrained by measurements of the Hubble constant made to higher precision than was possible with the first generations of Hubble Telescope instruments. A streamlined distance ladder constructed from infrared observations of Cepheids and type Ia supernovae with ruthless attention paid to systematics now provide <2% precision and offer the means to do much better. By steadily improving the precision and accuracy of the Hubble constant, we now see evidence for significant deviations from the standard model, referred to as LambdaCDM, and thus the exciting chance, if true, of discovering new fundamental physics such as exotic dark energy, a new relativistic particle, or a small curvature to name a few possibilities. I will review recent and expected progress. Should you wish to order lunch, you may do so here <https://forms.gle/dtBfQU6s6hCpJBnA7> *by 10am on Thursday*, October 17th.

# Gravity Group Lunch Seminar - Added

## The Expansion of the Universe, Faster than We Thought

### Featuring

Adam Riess

### Speaker Affiliation

Johns Hopkins University & Space Telescope Science Institute

### Affiliation

Natural Sciences

Date & Time

October 18, 2019 | 12:00 – 1:00pm

### Location

Jadwin Hall, Joe Henry Room, Room 102