Saturday, November 30, 2013

Dark Energy Part 3

Two NASA Pie Charts Tell Us a Great Deal - And Bring Up Many Questions

I used the two pie charts below left in a previous article, Dark Matter. We'll use them again as we explore dark energy.

The percentages came from analyzing Wilkinson Microwave Anisotropy Probe (WMAP) data, launched in 2001 to measure minute differences in temperature in the cosmic microwave background (CMB) across the full sky. The analysis of this data was completed in 2012. The percentages represent the relative contributions of various kinds of matter and energy to the overall mass-energy of the universe (remember that mass and energy are ultimately equivalent). The charts look deceptively simple but when you begin to look more closely they are not. First, it is hard not to notice the very large component of dark energy in the universe today, and yet there was almost none or none in the very young universe (380,000 years old). Where did it come from?

Most physicists believe that our universe is a perfectly isolated system in terms of thermodynamics. This means that whatever energy the universe started out with, that is how much energy it contains today. There are some theories about our universe that challenge this assumption - more than one universe may exist and there may be some kind of transfer of energy between our universe and other universes. You will come across versions of this multiverse theory when you examine black holes and theoretical white holes and when you examine theoretical possibilities for what underlies the mysterious force of gravity. Here, we will consider it to be isolated. That means that the pie charts are accurately the same size - each represents the same total amount of mass-energy. For now, I will leave this question of thermodynamics for a moment as we examine the pie charts in more detail. However, as we will see later on, the very notion of conservation of energy in an isolated system will be challenged by dark energy.

The Pie Charts - The Mysterious Evolution of the Universe

If we added a pie chart for the universe the micro-micro-micro second it popped into being it might consist of just one colour representing the undefined "mother" energy most physicists believe existed before the process of symmetry breaking took place (you can investigate this process further in the Gauge Theory article). This "mother energy" idea is defined by the Theory of Everything. In this pie chart there would be no matter, no photons, no neutrinos and possibly no dark matter or dark energy either.

When you first look at these pies, you might assume that most dark matter simply converted to dark energy over time. Their contributions seem somewhat similar. Importantly, the two are not related to each other in any way except that they are both mysterious - no one knows for certain what they are and what mechanisms are behind them - and they both contribute significantly to the current mass-energy of the universe and how it operates.

Notice the mysterious reduction in the light blue "atoms" slice. We will tackle the mystery of where the atomic contribution to the universe went, next, in Dark Energy Part 4.

Dark Energy Part 2

What Expansion of Space Means

It might seem obvious that expanding space is simply objects such as stars moving away from each other. This assumption, however, is not correct. The expansion of space is not like an explosion on Earth, for example, where pieces fly apart and the volume of debris expands while the underlying space stays constant. Space is measured as a metric tensor that changes over time (to understand what a tensor is, see the article Gauge Theory where it is explored). This means that distance is not a fixed measurement. It's not that stars are moving at all; space is expanding in between them. If a tape measure could be strung between two stars that are stationary with respect to each other, the actual markings on the tape measure would spread out, the scale would change in other words, showing the growing space between them. This is the subtle but often overlooked metric nature of space.

The following 5.5 minute video called "What is the Universe Expanding Into?" is by Deep Astronomy:

Two periods of accelerated expansion mark the history of the universe. The first acceleration, cosmic inflation, took place at an astonishing rate. Some models suggest that the universe doubled in size every 10-35 seconds.  The second, current, acceleration is even more stupendous. The volume may now be doubling at a rate about 50 times higher than that of cosmic inflation.

No one knows why the expansion rate is increasing. This is one of the most fundamental puzzles in physics today, and it is where the dark energy story begins.

One thing is certain: The universe is far more mysterious to us today than it has ever been. Twenty years ago, when we gazed at the night sky with our best telescopes, we saw the vastness of distant stars, galaxies and glowing nebulae, and we thought that, with the exception of dark material such as planets, brown dwarfs, non-glowing gases and black holes, we could see everything out there. Dark matter and dark energy tell us that the visible universe is a mere fraction of what really exists.

Up next: Dark Energy Part 3.

Friday, November 29, 2013

Dark Energy Part 1

How Dark Energy Fits Into the Evolution of the Universe

The exploration of dark energy is very au courant. And it is truly an odyssey, one that pries layers back to the very core of what spacetime is. It asks us to explore what the universe is composed of and how its behaviour has evolved. There are necessary sidebars to this story, many of which present additional mysteries. Therefore, dark energy has been written as a miniseries of articles.

It's very well established in science that the universe came into existence with a Big Bang. Since then, the universe expanded and matter clustered into gas clouds, stars, and galaxies, shown below.

The universe can be extrapolated back to a single point of origin about 13.8 billion years ago ? an infinitely tiny space filled with an almost infinite density of energy. The abundance of light elements in the universe, along with the existence of cosmic microwave background (CMB), as well as the Hubble's law (more about this later on) and what scientists know about the nature of large-scale structures, such as galaxies, all point to a single point of origin filled with infinite density. This tiny ultra-dense point immediately and rapidly expanded, cooling as it did so. After an initial brief phase of extremely rapid expansion, called cosmic inflation, lasting only a tiny fraction of a second, the universe was seeded with electrons and quarks - the first particles of matter that would later combine to form every structure in the universe today from gas clouds to stars to planets, trees and us. 380,000 years later, photons began to decouple from electrons and stream outward in all directions. These photons are what the CMB is made of. This fascinating "baby period" of the universe is explored in the articles, Our Universe Parts 2 through 11, starting here.

The universe experienced massive shifts in its energy composition as it evolved from a tiny space filled with pure dense undefined energy into an incredibly large space filled with an astonishing assortment of energy and matter particles - those which underlie all the forces and kinds of matter that are observed today.

First Glimpse of Something Amiss: Accelerating Expansion?

Our present universe is brimming with matter, and we know that matter is attracted to matter through the force of gravity. Until the late 1990's, scientists therefore assumed that the universe's rate of expansion must be decreasing as gravity pulls all matter together.

Then, observations of extremely distant galaxies by the Hubble Space Telescope began to question that basic assumption, as evidenced in this 1999 NASA press release. Every galaxy observed appeared to be moving 260,000 km/h faster for every 3.3 million light-years away from Earth.

Around this time, observations of distant Type 1a supernovae blew the assumption right out of the water. Hubble observations proved that the velocity at which distant regions of space are moving is increasing with time.

This means that the rate of expansion of space is increasing rather than decreasing, as indicated by an outward curve in the NASA diagram below, one that has not been repeated since cosmic inflation.

(Credit: NASA: Ann Field (STScl)
We will explore what expansion of space means, next, In Dark Energy Part 2.