# Dr. Anna Ijjas

## What is your current position at our institute?

I am the leader of the Lise Meitner Research Group “Gravitational Theory and Cosmology”.

## What is your academic education?

- 2008. Staatsexamen in Mathematics and Religious Studies, Ludwig-Maximilians-Universität München
- 2009. Staatsexamen in Physics, Ludwig-Maximilians-Universität München
- 2010. PhD in Philosophy of Quantum Physics, Ludwig-Maximilians-Universität München
- 2014. PhD in Theoretical Cosmology, Humboldt Universität zu Berlin

## What were your previous academic positions?

- 2014-17. John A. Wheeler Postdoctoral Fellow. Princeton Center for Theoretical Science, Princeton University, Princeton, NJ (USA)
- 2017-19. Simons Fellow (with the university rank of Associate Research Scientist). Institute for Theory and Computation, Harvard University, Cambridge, MA & Center for Theoretical Physics, Columbia University, New York, NY (USA)

## Can you please describe your research?

Did our universe have a beginning? Will it ever end? What is space-time like in the interior of black holes? In my research, I target the big open questions of cosmology. I combine novel theoretical ideas with modern techniques of mathematical and numerical general relativity and beyond, with the ultimate goal of making these questions empirically testable.

## What is your favourite figure from a paper you co-authored?

We propose *a new kind of cyclic universe* in which there is no big bang and no inflation but, instead, all the observed properties of the universe are the result of regularly repeating periods of expansion and contraction, driven by dark energy that causes space to grow exponentially in size from cycle to cycle.

The concept of a cyclic universe is old, but earlier attempts had the problem that, during contraction, space squeezes down so much that matter – including all the stars and galaxies – crunch together into a dense “mess” that may prevent the universe from “bouncing” over to the next expanding phase. The novelty of our ansatz is that it has a period of contraction that smooths the distribution of energy and flattens space, in accord with observations, but there is no crunch. Hardly any contraction is needed to match what we observe. In fact, there is more expansion than contraction in each cycle, which is why space grows exponentially from cycle to cycle.

As the figure illustrates, in the new cyclic model, the Hubble parameter H(t) oscillates (upper panel) with an amplitude, say, of ∼10^{10} GeV over a period t_{cyc}. The scale factor a(t), by contrast, is not periodic. Rather, it goes through long periods of expansion followed by shorter periods of contraction. The net result is an exponential increase in a(t) over the course of each cycle, producing an on-average de Sitter-like expansion over many cycles. The thin filled rectangular tickmarks indicate the cross-overs from contraction to expansion due to a (non-singular) classical bounce. The thin unfilled rectangular tickmarks indicate the cross-overs from accelerated expansion to ekpyrotic (ultra-slow) contraction.

Anna Ijjas' personal webpage can be found at http://www.anna-ijjas.com and her group's website is https://www.aei.mpg.de/gravitation-and-cosmology.