This article is dedicated to the memory of a great scientist, my cosmological mentor, and my very dear friend, David N. Schramm. David had been scheduled to face Jim Peebles in this Great Debate; after his tragic death in a plane crash last December, I agreed to take his place in this event which is now dedicated to his memory. Throughout his career David was ``bullish'' on cosmology and for a number of years he had been speaking about the coming Golden Age in cosmology, where a flood of cosmological data would test the bold ideas that blossomed from the connection between the Inner Space of elementary particles and the Outer Space of cosmology which he helped to pioneer. I am certain that he would have enjoyed his role in this celebration of cosmology, and my hunch is that he would have answered the question, ``Cosmology Solved?'', with the answer that I have.
Cosmology is the scientific study of the origin and evolution of the Universe, and the word itself derives from the Greek, cosmos, meaning order. From my perspective as a particle cosmologist, I would say that Cosmology is Solved when we explain and understand the basic features of the Universe, those which define its fundamental character, in terms of a theory rooted in fundamental physics.
Solving cosmology does not mean the end of the study of the Universe, nor even the beginning of a less exciting period of astrophysical inquiry. An analogy may be helpful; we have known the laws of quantum mechanics for more than sixty years, and quantum physics continues to be a vibrant field of study, as evidenced by recent advances including the first Bose - Einstein condensates of atoms, high-temperature superconductivity, fractional quantum Hall effect, quantum computing, and quantum interference devices.
The Universe is the most amazing and wondrous ``zoo'' one can imagine, full of all kinds of interesting objects and a diversity of phenomena. Astrophysics is the scientific pursuit of an understanding of these objects and phenomena in terms of the laws of physics. It is difficult to imagine astrophysics ever being solved. A list of today's puzzles is challenging enough to occupy astrophysicists for decades: What are the objects that make gamma-ray bursts and how do they work?; How do galaxies form stars and light up the sky?; How are stars born?; When were the first stars born?; What mechanism makes stars explode as supernovae?; Most of the ordinary matter is not in the form of stars, but is dark - what is it?; How do planets form?; Is there life elsewhere in the Cosmos?; How do massive black holes form? What is the origin of the highest energy cosmic rays? and on and on. As we are flooded with data from new ground-based and space-based observatories and experiments in the coming years and some of these questions are answered, the list will grow longer, with new, more interesting questions being added. Cosmology solved or not, I am confident that there will be plenty of challenges for next century's astrophysicists.
The revolution in cosmology triggered by the discovery of the CMB in 1964 led to the establishment of the hot big-bang cosmological model as the standard cosmological model (see e.g., Silk 1980; or Peebles et al. 1991). I believe the hot big-bang theory will be viewed as one of the great intellectual triumphs of the 20th century. Based upon a simple mathematical model, the Friedmann - Lemaitre - Robertson - Walker (FLRW) solution of Einstein's equations, it describes accurately the evolution of the Universe from a fraction of a second after the bang until today. As discussed by Silk, the FLRW model stands upon three experimental pillars: the observed expansion of the Universe; the existence of the cosmic microwave background (CMB) radiation; and the abundance pattern of the light elements D, 3He, 4He, and 7Li produced seconds after the bang in a sequence of nuclear reactions known as big-bang nucleosynthesis (BBN).
As successful as the FLRW cosmology is, there are a number of fundamental questions that it leaves unexplained. Here is the list of questions that I believe must be addressed before we can say ``Cosmology Is Solved'':
Origin of the expansion and definitive measure of the present expansion rate H0 (Hubble's constant).
Origin of the heat in the Universe and a precise measure of the present temperature of the CMB.
Full accounting of matter and energy in the Universe. From such an accounting one can infer the present rate of deceleration (or acceleration) of the expansion and the geometry of the Universe.
Understanding of the origin of the density inhomogeneities that seeded all the structure seen in the Universe today.
Understanding of the origin of ordinary matter and particle dark matter.
Understanding of the dynamite behind the big bang. The term ``big-bang theory'' is a misnomer - it is not a theory of the big-bang event, but rather, of the events thereafter.
Understanding of the regularity of the Universe, as evidenced by the uniformity of the CMB (temperature variations across the sky of less than one part and in 104 and the statistically homogeneous distribution of galaxies).
Description of the history of the Universe from the big-bang event on.
As I will discuss in detail in Section 3, Inflation + Cold Dark Matter is a theory that addresses all of these questions as well as extending our understanding of the Universe back to times as early as 10-32 sec. Its fundamental predictions are that the Universe is spatially flat, that the bulk of the matter exists in the form of slowly moving elementary particles (and not the stuff that we are made of), and that diversity of structure we see in the Universe today, from galaxies to the great walls of galaxies (Geller and Huchra 1989), arose from quantum mechanical fluctuations on subatomic scales.