The universe is expanding. We sit on our tiny blue world in, as Douglas Adams put it, an unfashionable Western Spiral arm of a galaxy. This galaxy drifts lonely and unregarded, pulled along by the expansion of spacetime. As we sit here and wonder, the universe in contemplation of itself, we regard our neighbor galaxies as they speed away from us in silence. Cosmically speaking, these galaxies do not just drift lazily in the tug of spacetime – that tug grows stronger every day, accelerating the expansion of the universe.
There have been several observations now that suggest the universe is not only **not** static (as Einstein believed it must be), and not only expanding, but expanding faster with each moment. It is as if some unseen force, a force whose impetus is derived from a dark energy source that fills the universe, is shoving more and more galaxies beyond our relativistic event horizon, faster and faster. Type Ia supernovae, whose fierce birth from stellar death and subsequent fade are like standardized candles winking across the void, speak to us of the acceleration of the universe at different epochs. Data on *high-redshift supernovae*, those which are most distant from us and thus furthest back in time (we see the light of their birth billions of years after they actually occurred, owing to the finite velocity of light), tell us that in the past the universe expanded more slowly than it does now. The echo of the universe’s birth, the distribution of matter and its density across the universe, speaks from the cosmic microwave background and can only be explained by a universe that was born with some inherent dark energy reserve which, in our current epoch, exerts a negative pressure on the universe.
Today, a new paper (promised by its researchers one week ago) appeared in the arXiv, the online physics paper repository. This paper summarizes the first year of data from the Supernova Legacy Survey (SNLS), slated to run a full five years [ASTROPH0510447]. This group used telescopes, and a “rolling search” technique, to identify 71 high-redshift Type Ia Supernovae. They then studied the Hubble diagram of these supernovae and compared that to models of the universe. For instance, they compared models with a dark energy component and a matter density component, models where the universe is expanding or collapsing, models where the curvature of spacetime is flat, open, or closed, and a model where the big bang did not occur. I reproduce below their Fig. 5.
What does this result tell us? First, let’s discuss the axes. The vertical axis represents the density of the universe as contained in dark energy (a non-matter component). The horizontal axis represents the density of the universe as contained in a matter components. For centuries, the assumption was that matter was the dominant component of the mass of the universe – that is, stars and planets and all other matter-particle-based structures caused all the mass in the universe. Now, let’s look at what the data tells us when interpreted in terms of these two densities (these axes).
First, the contours that look like ellipses stretching from the bottom left to the upper right represent the most likely place on this plane for our universe to lie, given the data. The outermost contour is the 99.7% probability contour, which means that the probability that the same experiment repeated over and over and over with unique data sets derived in the same manner would yield results outside the contour is only 0.3%. Statistically speaking, that means that our universe is most consistent with the data when it lies inside that contour. One thing becomes very obvious upon inspection: the big bang mus have occurred. The region that would be populated by universes without a big bang lies in the upper left-most corner, and is very far from the edge of the 99.7% contour. What I find remarkable is just how well this plot represents real science. The big bang theory makes predictions: the universe would have dark energy densities above 1 (and above our contours) if it had never occurred. Here we see this prediction born out by the data. Yet again, the big bang is verified.
These contours also lie well within a region where the universe would have to be expanding more rapidly, rather than slowing (or static). The region of a decelerating universe lies in the lower right corner of the plot, the region where the matter component of the universe must be closer to 1 and thus exerting maximal gravitational self-attraction.
There is more to see in this figure. To see it, we have to consider the impact of an independent data set from the baryon acoustic oscillations (BAO) as measured in the Sloan Digital Sky Survey (SDSS). The contours derived from those data select a region that goes from the upper left to the bottom middle of the figure. Since the BAO data is independent of the SNLS data, and both can be interpreted in this framework, we can combine them (see where their contours overlap) to further constrain our understanding of the universe. The overlaps contours are the small ellipses. From these contours, we see that the data favor a flat universe (one where there is no curvature to spacetime).
Remarkable. Seventy one distant supernovae speak to us across the vast cosmic silence, winking for only an instant when compared to the life of this ancient universe. They tell us of the stretching of spacetime that has occurred since their light left them, billions of years ago, when the galaxies in this universe were 98% closer than they are now. We read from their light the alarming fact that this universe is not only dominated by a source of energy outside of matter, but that this energy is exerting a negative pressure that is accelerating spacetime’s expansion. We sit on the edge of this void at a remarkable moment in the universe’s history, and we ponder.
.. [ASTROPH0510447] “http://arxiv.org/abs/astro-ph/0510447”:http://arxiv.org/abs/astro-ph/0510447
