Adventures in Cosmology

The following sections are included:

• CONTENTS

Many non-scientists tend to think that cosmology became a science only in our own time, prior to which it was a vague system of beliefs that did not amount to much. But in fact the Greeks were cosmological scientists (Plato, Aristotle and many who predated those two), and the tradition continued with Ptolemy in second-century Alexandria, and especially with the likes of Copernicus, Kepler, Galileo, and Isaac Newton. Each of those thinkers had a vision of how the Universe works and tried to arrange the empirical facts to fit that vision. That is what cosmology is all about. This book presents essays on the current state of the art by their modern counterparts…

It was once observed¹ that galaxy formation “is a process, not an event”, meaning that galaxies began forming rather quickly (by cosmological standards) after the Big Bang and are still forming today. Galaxy formation is a consequence of the remarkable ability of gravity to organize matter over cosmological scales, even in the face of cosmic expansion. In this chapter, we will explore the physical processes that control the process of galaxy formation and how they shape the properties of galaxies, both at the present day and in the early Universe.

Cosmology is by now a mature experimental science. We are privileged to live at a time when the story of genesis (how the Universe started and developed) can be critically explored by direct observations. Looking deep into the Universe through powerful telescopes, we can see images of the Universe when it was younger because of the finite time it takes for light to travel to us from distant sources.

Existing data sets include an image of the Universe when it was 0.4 million years old (in the form of the cosmic microwave background), as well as images of individual galaxies when the Universe was older than a billion years. But there is a serious challenge: in between these two epochs was a period when the Universe was dark, stars had not yet formed, and the cosmic microwave background no longer traced the distribution of matter. And this is precisely the most interesting period, when the primordial soup evolved into the rich zoo of objects we now see.

The observers are moving ahead along several fronts. The first involves the construction of large infrared telescopes on the ground and in space, that will provide us with new photos of the first galaxies. Current plans include ground-based telescopes which are 24–42 m in diameter, and NASA's successor to the Hubble Space Telescope, called the James Webb Space Telescope. In addition, several observational groups around the globe are constructing radio arrays that will be capable of mapping the three-dimensional distribution of cosmic hydrogen in the infant Universe. These arrays are aiming to detect the long-wavelength (redshifted 21 cm) radio emission from hydrogen atoms. The images from these antenna arrays will reveal how the non-uniform distribution of neutral hydrogen evolved with cosmic time and eventually was extinguished by the ultraviolet radiation from the first galaxies. Theoretical research has focused in recent years on predicting the expected signals for the above instruments and motivating these ambitious observational projects.

The following sections are included:

• What are Galaxy Clusters? Why are They Interesting?

• Structure Formation

• How do We Observe Clusters?

• Clusters in Cosmology

• Dark Matter or Modified Gravity?

• Gas, Galaxies and Their Evolution

• Acknowledgments

• References

Recently, cosmologists have succeeded in determining the fundamental parameters of our cosmic world model with extremely high precision. There remains, however, one crucial gap in our understanding: How and when did the first stars and galaxies form? Their emergence during the first billion years had dramatic consequences for the history of the Universe, leading to a phase-transition from a cold, neutral medium to an almost completely ionized, hot one. With the help of cutting-edge observatories and supercomputer simulations, we are about to lift the veil that is still shrouding the epoch of cosmic dawn.

The following sections are included:

• A Brief History of Our Universe: From Soup to Galaxies

• The Hidden Cosmic Dawn

• The Solution: Flipping Spins

• The Spin-Flip Transition as an Astronomical Tool

• Foiled!: Early Cosmology with the Spin-Flip Transition

• Spin-Flip Radiation Holds the Key to Observing the Cosmic Dawn

• The Spin-Flip Background: The First Stars

• The Spin-Flip Background: The First Black Holes

• The Spin-Flip Background: The Epoch of Reionization

• FM Radio Antennae as Cosmic Observatories

• Piles and Tiles of Antennae: Mapping the Spin-Flip Background

• Mountains to Scale: Challenges to Observing the Spin-Flip Background

• Sound and Fury, Signifying Statistics

• An Explosion of Telescopes

• Dreams for the Future

• An Unfinished Story

One of the most exciting results in physics over the last decade has been the evidence that neutrinos have mass. In fact, so far neutrinos are the only form of non-baryonic dark matter that exists for sure. Particle physics experiments have confirmed that neutrinos oscillate between the three known flavors and this, in turn, implies a finite difference in their mass “eigenstates”. While there is strong evidence for new physics beyond the Standard Model, this does not determine the absolute value of the masses. Cosmology not only probes this absolute mass scale, by currently determining the sum of the masses, but it is a completely independent method for which to test against. We now know that their masses are smaller than a billionth of a single hydrogen atom and that they are so elusive that they can pass uninterrupted through slab of lead with a width of a light year. Naturally, this requires understanding of exactly how the mass is measured from cosmology. Weighing the neutrinos provides perhaps one of the most beautiful interplays between physics of the largest and smallest scales and in the process highlights the synergy between particle physics and our Universe.

The following sections are included:

• Introduction

• Twinkle Twinkle Little Star: Cepheid Variables
  • A brief historical overview
  • The theoretical perspective
  • The Leavitt law
  • An independent check
  • Hubble (the telescope) observes Cepheids

• The Aborted Explosion of Stars: The Tip of the Red Giant Branch

• Bumpy Galaxies: The Surface Brightness Fluctuation Method

• The Orderly Nature of Early Type Galaxies: The Fundamental Plane

• The (Not Quite As) Orderly Nature of Spiral Galaxies: The Tully–Fisher Relation

• Stellar Explosions: Type Ia Supernovae

• Further Reflections and Future Directions
  • Eighty years of Hubble constants
  • The age of the Universe
  • Room for improvement

• References

Although the nature of our Universe's dark matter remains a mystery, it is likely to consist of a new species of particles, perhaps associated with the electroweak scale. Such weakly interacting massive particles (WIMPs) are theoretically attractive in part because they can be thermally produced in the early Universe with a density consistent with the measured dark matter abundance. In this chapter, I will describe some of the most popular particle candidates for dark matter and summarize the current status of the quest to discover dark matter's particle identity, including direct and indirect searches for WIMPs, and efforts being pursued at particle accelerators.

The following sections are included:

• Introduction

• Direct Detection of WIMPs via WIMP–Nucleon Elastic Scattering

• Inelastic Scattering

• Background Sources

• Backgrounds and WIMP Sensitivity

• Direct Detection Techniques
  • Detectors without nuclear-recoil discrimination
    • Germanium spectrometers
    • Scintillation-mediated detectors
  • Detectors with nuclear-recoil discrimination
    • Metastable or threshold detectors
    • Sub-Kelvin detectors
    • Liquid noble detectors
    • Two-phase detectors
    • Single-phase argon detectors
  • Direction-sensitive detectors

• Acknowledgments

• References

In this article we review the discovery of the accelerating Universe using Type Ia supernovae. We then outline ways in which dark energy — component that causes the acceleration — is phenomenologically described. We finally describe principal cosmological techniques to measure large-scale properties of dark energy. This chapter therefore complements articles by Caldwell and Linder (2010) in this book who describe theoretical understanding (or the lack thereof) of the cause for the accelerating Universe.

The following sections are included:

• Introduction to Dark Energy

• The Dynamics of Nothing

• Knowing Nothing

• The Frontiers of Nothing

• Conclusions

• Acknowledgments

• References

The following sections are included:

• Supermassive Black Holes and Galaxy Formation

• Observations of the Most Distant Quasars

• Growing the First Supermassive Black Holes in the Universe

• Future Prospects

• References

The following sections are included:

• INDEX