Change in Galaxies and the Universe Itself--Interactive Text
Earth Science Extras
by Russ Colson
Examples of the three main types of galaxy, Spiral, Elliptical, and Irregular, imaged by the Hubble Space Telescope (courtesy of NASA, ESA).
Galaxies Change
As of this writing, no theory for the evolution of galaxies can account for all of the observations of the shape and distribution of galaxies in our universe. This does not mean the theories for galaxy evolution are entirely wrong, but it does mean that there must be details that we don't yet fully understand. Rather than providing an introductory lecture on this topic, I am sending you out into the internet to learn about what we know so far about the evolution of galaxies. For example, you might check out https://en.wikipedia.org/wiki/Galaxy_formation_and_evolution, and https://www.britannica.com/science/galaxy/Evolution-of-galaxies-and-quasars. When you have read these, try tackling the questions below to see if you have extracted some of the key ideas.
Regardless of any uncertainty in our theories, the existence of a wide variety of different types of galaxies, that is, galaxies of different morphologies and star spectra, suggests that either these galaxies formed by different processes, from different starting states, or they have changed due to subsequent processes of evolution. Both of these might be true to varying degrees. Evidence for how galaxies formed and evolved is found 1) in the nature and distribution of the galaxies we see around us (perhaps similar to how we looked at the distribution of stars ito inform us about their life cycle in a previous lesson), 2) in the character of the very distant galaxies (which, due to the limitations of the speed of light, is like looking back in time to what galaxies were like in the early days of the universe), and 3) in computer simulation of how galaxies might form or interact.
A detailed examination of current theories of formation and the evidence for each is beyond the scope of this lesson. However, the challenges below present some of the observational and modeling evidence for you to consider.
The Big Bang and Red Shift
Learning to investigate a problem by researching what other people have already learned about it is an important part of both research and teaching. Before reading the text below, do some reading online about the Big Bang and Red Shift. When you are ready, return to the text below at test how many key ideas you were able to figure out from your own online reading.
Redshift: The first evidence for a Big Bang.
Hopefully in your readings you figured out the basic idea of doppler redshift: if a source of light is moving away from you, the wavelength of the light will be shifted toward longer values. This is similar to how the sound of a truck or train changes as it approaches and passes you. When approaching, the sound waves get scrunched up, the wavelength is made shorter, and the frequency is higher. But when the same truck or train is moving away from you, the waves get stretched out, the wavelengths are longer, and the frequency is lower. Red shfit is a shift toward longer wavelengths of light when something is moving away from you, similar to the longer wavelengths of the sound waves from the train.
Hopefully you also figured out that the cosmological redshift is not quite the same things as a doppler redshift. When thinking about the redshift of light from a distant galaxy, it is better to think of the universe between us and the distant galaxy as expanding, causing an expansion of the light waves in between, rather than thinking of the galaxy moving away from us. Think of two balls (galaxies) glued to a big rubber sheet (the universe). When you stretch the sheet, the balls get farther apart even though neither of them is moving relative to the spot on the rubber sheet where they are glued. If we imagine the balls to be on the sheet, but not glued to it, we can understand that the balls could both move relative to the sheet (causing doppler red or blue shift) and also the sheet could stretch out (causing cosmological redshift).
If you imagine three balls glued to the same rubber sheet, when the sheet is stretched the farther-apart balls will move apart faster. This is what we observe in the universe, farther-apart galaxies are moving away from each other faster. Since everything is moving apart, we can imagine unstretching the universe and projecting back to a time when all the galaxies were together at one place. It was from this moment and place that the galaxies began their journey apart--the Big Bang.
Hopefully you also figured out that it doesn't really work to think of the Big Bang as an explosion. An explosion throws material out in an expanding sphere, but material from an explosion would not continue to accelerate as it got further from the explosion. Not would you have a situation where every object recedes from every other object in such a way that the speed of separation gets higher with distance. The relationship between separation rate and distance requires that space itself is explanding--thus more like our stretching-rubber-sheet model than our balls-moving-apart-independently model. Another failure of the explosion model is that we observe redshift values that correspond to a speed greater than the speed of light (if we assume a 'doppler shift' type model in which objects are physically moving apart from each other). Relativity constraints suggest that a galaxy cannot be moving away from us faster than the speed of light. However, if objects are moving apart because space itself is expanding, this lightspeed limitation does not occur.
This discovery that the Universe itself is changing, expanding continuously like a rubber sheet between the galaxies, came relatively late in the history of scientific "discoveries of change". The realization that earth, life, and even the stars change developed from the early 1800s, but the realization that the entire universe is changing grew in the 1920s and 30s. The conclusion that we can project the universe back to a beginning moment, a Big Bang, came somewhat concurrently with the idea that the Earth had a beginning.
The early realizations about the Earth's immense age and the change that happened to it came from the stratigraphic studies of James Hutton and Charles Lyell, examined in other lessons. Those two geologists did not necessarily buy into the idea that there was a clear beginning point in geological time. James Hutton had concluded in 1788 that the Earth had "no vestige of a beginning, no prospect of an end." You may remember from previous lessons how Lyell in the 1800s struggled with the idea of progression in evolution. He preferred to think of a world that, although clearly changing, was perhaps changing in a somewhat cyclical way.
The idea of that Earth had a beginning originated with the discovery of radiometric dating in the early 1900s. People realized that if we could measure a firm age for the Earth, that meant that there was a start to it all. This realization emerged somewhat concurrently with the idea in the 1920s and 30s that the Universe had a beginning, a 'Big Bang." This idea, that there was a beginning, was a philosophically profound one in the sciences where ideas of cyclicity of change versus an arrow to change had been conflicting paradigms for some time.
Hubble and the Expanding Universe
The graph below is taken from Edwin Hubble's 1929 paper, A Relation between Distance and Radial Velocity among Extra-Galactic Nebulae, Proceedings of the National Academy of Sciences, vol. 15, no. 3, pp. 168-173. This is one of the earliest studies that seem to confirm that the universe is expanding. It is one of the foundations for the Big Bang theory. In this graph, the Y axis plots velocity relative to Earth in kilometers per second, and the X axis plots distance from Earth in Parsecs (106 parsecs = 1 mega parsec, 2 x 106 parsecs = 2 megaparsecs). Hubble based his measurement of distances from Earth on a variety of different standard candles, a principle examined in a previous lesson (Measuring the Sky).
Hubble's first estimate gives us an age for the universe that is significantly younger than the age of the Earth measured by geologists--quite a problem, sort of like if you discovered that your bones were significantly older than you were (or worse, discovered that your biological mother was younger than you)! But as time passed, better measurements were made of the distances to galaxies, which provided better estimates for the Hubble Constant. It was also realized that universe expansion may have changed through time, which also had an effect on the estimate for the constant.
Background cosmic radiation, the second big evidence for the Big Bang
When we look up at the sky, day or night, we see light. The light is not evenly distributed, there being more intense light during the day than at night. There is more intense light if we look toward the Sun, or Moon, or a star, or toward the milky brightness of the Milky Way core than when we look in directions away from any of these. This anisotropy of light is also true, in general, for other types of electromagnetic energy. The intensity of X-rays and infrared, for example, depend on the direction we look and when we look there.
Suppose, though, that you found a source of radiation that was almost the same regardless of whether it was day or night, or where in the sky you looked?
That's the kind of energy that cosmologist Ralph Apher and others predicted in 1948 should result from the Big Bang. Actually, the energy would come from some time after the big bang when the universe cooled off enough so that energy could start to move through it. Before that, the energy was absorbed by the free protons and electrons. The energy would be everywhere, coming (almost) equally from every direction. It would also be of fairly long wavelength because the energy has been travelling for so long through expanding space (remember the stretching rubber sheet?) that the original shorter wavelength has been stretched out so that it resembles the waves generated by a very cold black body radiator (remember black body radiation from our previous unit).
In the early 1960s, scientists at Bell Laboratories were working to perfect a highly sensitive antenna in order to detect faint microwave signals bounced off of shiny Echo balloon satellites as an early method of sending signals around the world via satellite.
As a side note: These satellites were very shiny and highly visible and could be seen, at one time or another, from almost anywhere on Earth. I remember that on one summer evening in rural Kansas where I grew up my family spotted a bright light like a particularly steady star streaking north through the sky and my Dad said "I bet it's one of those new-fangled satellites!"
As part of their work, the Bell scientists had to remove all sources of background noise so that they wouldn't miss the faint microwaves echoing off of the Echo satellites. They accounted for noise generated by radio broadcasting and radar use. They cooled the components of the antennae with liquid nitrogen so that radiation from the antennae itself wouldn't overwhelm the signal.
Radiation coming from the atmosphere (it's a blackbody radiator!) was taken into account by measuring how the amount of electromagnetic "noise" changed as the antenna was rotated from the horizon upwards, with different amounts of atmosphere so that a correction for atmosphere at any partiuclar elevation could be made.
Noise can also arise from loss of signal due to ohmic (resistance) loss and leakage. These sources were calculated and taken into account.
But they still found noise in the signal.
To make sure the extra electromagnetic noise wasn't coming from the ground (remember our black body radiation lessons--earth radiates microwave energy), they put sources of electromagnitic "noise" around the antennae and moved the source and the antenna around to see if there was significant input from that source. That didn't account for the extra noise in the signal.
The scientists considered the possibility that some noise might arise due to impoerfect connections across seams in the antenna. To test for this, they taped the seams with conductive aluminum tape and observed that there was no significant change in noise, suggesting this was not a major source of noise.
After all the sources of noise were accounted for, there was still more noise. This noise did not change from day to night. It did not change with season, It did not change as a function of where the antenna was pointed.
This proved that the noise was isotropic, coming from everywhere equally. Just as predicted for the energy remnants of the Big Bang.
The energy was in the microwave range (the Bell scientists examined waves of 7.35 centimeters wavelength), just as predicted for the stretched-out energy from the Big Bang.
The Bell Scientists reported their work, which eventually won a Nobel Prize in physics. Penzias, A. A. and Wilson, R. W. (1965) A Measurement of Excess Antenna Temperature at 4080 Mc/s. Astrophysical Journal, vol. 142, p.419-421.
Sometimes the best discoveries are found when we are trying to figure out something else, or when some mysterious problem we are trying to fix just won't go away!
Since the mid 1960's, improved measurements of the csomic background radiation (CBR) have found tiny, subtle variations that are consistent with what theoretical modeling of the radiation would expect, continuing to provide more evidence that this radiation comes from the Big Bang, and thus providing evidence that three was a Big Bang.
last updated 8/13//2020. The images of galaxies come courtesy of the Hubble Space Telescopr (NASA, ESA). The graph is from Hubble et al (1929) as cited. Other text and pictures are the property of Russ Colson.