This document summarizes the history of discoveries about the expanding universe and the unexpected findings that have challenged our understanding, including the accelerating expansion driven by dark energy. It discusses early models proposed by Einstein and others, the evidence from the cosmic microwave background and supernovae observations in the late 20th century that revealed the universe is expanding and accelerating, and outstanding questions about dark energy and dark matter that remain areas of active research today.

1. The Ever Surprising Universe
Levon Pogosian
Simon Fraser University

2. 100 years of General Relativity
“Spacetime tells matter how to move; matter tells spacetime how to curve.”
John A. Wheeler (1911-2008)
einstein.stanford.edu

3. Einstein’s Perfect Universe of 1917
Same Everywhere
Ever the Same
no evidence for this in 1917,
but turned out to be correct
he tried,
turned out to be wrong

4. Einstein’s Static Universe
aE
kE>0
speed of the
expansion
acceleration of
the expansion
A. Friedmann, “Über die Krümmung des Raumes”, Zeitschrift für Physik (1922)

5. Meine größte Eselei!

6. Implication of the expansion:
the universe was denser and hotter in the past
We can see the evidence:
looking deep into space = looking back in time

7. Us in the center

8. 1965: A. Penzias and R. W. Wilson of Bell Labs
1978 Nobel Prize in Physics
Discovery of the Cosmic Microwave Background

9. 1992: COBE detects
the tiny fluctuations in CMB temperature
that seeded the stars and galaxies
John Mather and George Smoot, 2006 Nobel Prize in Physics

10. Credit: NASA

11. Need dark matter for this to work,
which by then was old news …

12. Fritz Zwicky (1898 –1974)
Gravitational mass of the galaxies within the Coma cluster are
at least 400 times greater than expected from their luminosity.

13. The Standard
Cold Dark Matter
model
5% normal matter
95% dark matter
1992-1998
Two “minor” problems
a) only 10%-50% of the matter is accounted for
b) some stars are older then the universe

14. 1998: the expansion is accelerating!

15. http://supernova.lbl.gov/video.html
Standard Candles: Supernovae Type IA

16. Saul Perlmutter’s idea:
Look at thousands of galaxies for 2-3 nights every 2-3 weeks with
a wide-field imager. Follow up with spectroscopy, scheduled in advance
Saul Perlmutter starts the Supernovae Cosmology Project in 1988
Brian Schmidt starts the High-Z SN Search Team in 1994
Adam Riess leads the analysis for HZT
By 1998, SCP reported 42 reliable SNIa, HZT added another 16
Supernovae on Demand

17. A. Riess et al, Astron.J.116:1009-1038 (1998) S. Perlmutter, Astrophys.J.517:565-586 (1999)

18. S. Perlmutter, Astrophys.J.517:565-586 (1999) A. Riess et al, Astron.J.116:1009-1038 (1998)
70% of the universe is Dark Energy!

19. Since 1998
Spectacular CMB measurements by WMAP, Planck and other experiments
Millions of galaxy redshifts and shape distortions by gravitational lensing
1000+ supernovae, compared to 42+17 in 1998

20. Acceleration is Beyond Reasonable Doubt

21. We have a successful working model of cosmology …

22. … but there are reasons to keep an open mind
Observed vacuum energy:
Vacuum energy from known particles:
Something is NOT right!
And, we still don’t know what dark matter is!

23. https://www.spacetelescope.org/news/heic1908/

24. Could it be that the Dark Energy density is not constant?
Y. Wang, G.-B. Zhao, LP and A. Zucca, 1807.03772, Ap J Lett

25. Despite the many successes of the current model,
we should keep an open mind!
The universe surprised us before… nobody asked for
the expansion, dark matter, acceleration…

26. What will be the next surprise?
Questions we are asking today
Is Dark Energy changing with time?
Does General Relativity work outside our solar system?
How does the vacuum energy gravitate?
What is Dark Matter?
Do Dark Matter and Dark Energy interact with each other?
SFU is searching for answers as a member of Simons Observatory and CMB-S4
C O L L A B O R AT I O N C O U N C I L
SIMONS OBSERVATORY
Simons Observatory