Cosmic Times

Baby Universe's 1st Picture

The primary message of this article is that the discovery of tiny variations in the cosmic microwave background was a watershed event. Astronomers had known that this remnant radiation from the Big Bang could not be infinitely smooth. This is because we have a "lumpy" universe now, with clumps of matter that form structures we see today. The discovery of the anisotropy confirmed astronomer's ideas about the Big Bang.

Helping your students understand the concepts

The ideas presented in this article may be easier for your students to understand if they have first read and discussed the 1965 Cosmic Times article on the cosmic microwave background, Murmur of a Bang. The 1965 Cosmic Times lesson plan titled "Cosmic Microwave Background" may also help your students develop a solid understanding of the CMB.

In addition, we suggest that the lesson plan titled "What's the Problem with Isotropy" used after reading this article may help students understand the main concepts presented in this article.

The COBE satellite

The 1965 Cosmic Times introduced the observation by Penzias and Wilson of a radiation background left over from a time just after the Big Bang bathing the entire Universe. As with any astronomical discovery, the first observations serve to prove that a phenomenon exists. Astronomers scramble after such a discovery to develop methods and technologies to study the phenomenon in greater detail. The cosmic microwave background (CMB) is no different. NASA solicited for proposals for small- or medium-sized space missions in the early 1970s, and received three independent proposals to study the CMB. This sent NASA the message that the astronomical community wanted to study the CMB in greater detail, and that they had many ideas about how to carry out those studies. NASA responded by selecting members from each of the three teams to join together for a unified mission. The end result was Cosmic Background Explorer (COBE), which launched in 1989.

The three instruments COBE carried were designed for two primary tasks: measuring the CMB spectrum and mapping the CMB. One instrument made a detailed spectrum of the CMB. The other two instruments mapped the CMB across the entire sky.

The COBE spectrum

Using just the first 9 minutes of data from the spectrometer, COBE scientists were able to make a spectrum of the CMB. Shown below.

Spectrum from the first 9 minutes of COBE data

This is the spectrum that John Mather presented at the January 1990 meeting of the American Astronomical Society Meeting. It is the spectrum based on the first 9 minutes of data from COBE. The solid line shows a theoretical blackbody, and the squares show the COBE data. The error bars for the data are contained within the squares. (Image credit: Mather et al. Astrophys. J. Lett. , 354:L37ŠL40, May 1990)

One thing to notice about the plot is that the theoretical blackbody (solid line) and the data (squares) match precisely. Astronomers rarely see data and theory matching so precisely. In fact, when John Mather presented this very plot at the 1990 meeting of the American Astronomical Society Meeting, it was greeted with a standing ovation.

This spectrum was the first result from COBE, and it was enough to make some astronomers a little nervous. Why is that? Because the data is too smooth. Let's take a quick inventory of the Universe around us. There are planets, stars, galaxies and cluster of galaxies. In a word, it's clumpy. In other words, the matter is generally found in gravitationally bound structures.

The structure we see today must have come from seeds in the early Universe. But the CMB is a reflection of that early Universe, so those seeds must be present in the CMB or we would not see structure today. The smooth spectrum contained no hints of those seeds. The astronomical community would have to wait another 2 years to see if the maps made by COBE's other instruments contained the seeds that needed to be there to produce our present-day Universe.

The COBE Map

In 1992, George Smoot's team published COBE's map of the CMB, and for the first time astronomers saw the "lumps" in the CMB. These anisotropies, as astronomers call them, were at the very limit of COBE's capabilities, but were large enough to eventually form the structure that we see in the Universe today. The map is shown below:

Map of the CMB by COBE

COBE cosmic microwave background radiation (CMB) map. (Image credit: COBE Project, DMR, NASA)

The different colors show places where the very early Universe had temperature differences, which are equivalent to density differences. Higher density regions were the places where gravity started to form structures. We know that these anisotropies must be the seeds of structure that we see today, but the details of COBE are too coarse to identify specific seeds with present-day structures.

One important point is that while we talk about "lumps" in the early Universe, the size of those density differences was tiny. They represent changes of the order of 1 part in 100,000. Changes of this size are a bit like locating a single mosquito on a stretch of road a mile long. That's a small difference in density! In fact, the background was so smooth that other problems with Big Bang Theory arose even before these COBE results. However, these other problems are solved with inflation, as is discussed in the article titled "Inflation in the Universe".

Perhaps the most important point for students to walk away with is that on the one hand, we have very, very small changes in the background, but those changes turn out to be essential to the Universe we observe today. Big Bang theory would have been in a heap of trouble if astronomers hadn't seen those tiny changes in the CMB.

Other resources

The following web pages have more detailed information:

  • COBE website
  • If your school has an account with Discovery Education Online, they have several videos and clips discussing the cosmic microwave background and the expanding universe: Discovery Education Streaming
  • The Beyond the Solar System DVD has a few clips that deal with the cosmic microwave background and how the structure we see in the CMB were transformed into the Universe we see today. This DVD can be requested from the following web page.
A service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA/GSFC