# 2. COSMIC HISTORY

Distant galaxies seen in an extract from the Hubble Ultra Deep Field,
which records the faintest astronomical objects ever observed.

Astronomy is the only science that attempts to understand the nature of the universe as a whole (in empirical, not religious or mythological, terms). The study of the origin, evolution, and fate of the universe is called cosmology.

In the last lecture, as a way to provide context for the rest of the course, we had constructed a scale model giving a sense of the vast distances between the Sun and even its nearest neighbor stars. In this lecture, we extend our discussion to the largest measurable scales of time and space. Then we give a broad-brush overview of what we have learned so far about the evolution of the universe and its contents. This story is by no means complete, but we have a solid foundation to build upon.

## A. Billions and Billions

Yes, you really do need to use "billion-babble" in an astronomy class.
• One million is one thousand thousand, or, in powers of ten notation, 1,000 x 1,000 = 103 x 103 = 106.

• One billion is one thousand million, or, in powers of ten notation, 1,000 x 1,000,000 = 103 x 106 = 109.

• A billion of anything is very difficult to visualize. Visualizing a million is much easier: a million seconds elapse in only 11.6 days. But a billion seconds take almost 32 years. We will give some other examples in class.

• The scientific shorthand for quantities measured in billions of a given unit is "giga" --- so 5 billion years (the age of the Sun) is usually referred to as "5 giga-years" or "5 Gyr".

• The scientific shorthand for quantities measured in one billionth of a given unit is "nano" --- so, for instance, "nanotechnology" refers to things that have characteristic sizes of one billionth of a meter.

• A trillion is one thousand billion or one million million, or 1012 in powers of ten notation.

• See Supplement I for more information on powers of ten notation and units we will use in this course.

## B. Light as a Distance Standard

The scale of even the local universe is so huge that we would have to quote distances in trillions of ordinary units like miles or kilometers. Instead, astronomers sought a more convenient and more universal standard for measuring distances.

Light travels very fast but not infinitely fast. Its speed is 186,000 miles per second or 300,000 km per second (or about 1 foot per nanosecond). The speed of light has been measured in physics labs to a precision of about 1 part in a billion.

Furthermore, according to Einstein's Special Relativity the speed of light will always be found to have to same value for any observer in the universe as long as he/she/it is not accelerating through space. Also, in Relativity, no physical object can travel faster than the speed of light. Therefore, light speed is an excellent choice for a standard of velocity.

Accordingly, astronomers use the light travel time to objects as a measure of their distance. They characterize distances in terms of the time it would take a light ray to traverse that distance:

• For instance, we define a light second to be the distance a light ray travels in one second of time, which is 186,000 miles. The distance to the Earth's Moon is such that it takes light 1.26 second to cross it. Therefore, we can say the distance to the Moon is either "238,000 miles" or, equivalently, "1.26 light seconds". See illustration below:

The relative sizes and separation of the Earth-Moon system are shown to scale above.
The beam of light is depicted traveling between the Earth and the Moon in the same time it actually
takes light to scale the real distance between them: 1.255 seconds at
its mean orbital distance (surface to surface). (From Wikipedia.)

• The distance to the Sun is 8.3 light minutes.

• Even the nearest stars are much more distant. A convenient unit for typical stellar distances is the light year. One light year is the distance light travels in one year.

• A light year is about 6 trillion miles or 10 trillion km. That's 6,000,000,000,000 miles, or 6x1012 in powers of ten notation.

• For technical reasons, having to do with the practice of determining the distances to stars using the "parallax" method, astronomers more commonly use the parsec as a distance unit. One parsec is 3.26 light-years, so if you see "parsecs" quoted as a distance, just multiply by 3 to convert roughly to light-years.

• Here is a list of distances in light years to some other important astronomical objects.

Scale diagram of the Milky Way Galaxy (edge-on)
"You Are Here" marks the location of the Sun
(Click for enlargement)

## C. Our Galaxy and Beyond

### Our Local Stellar System: the Milky Way Galaxy

Alpha Centauri, the nearest star, is 4.2 light years distant. It is over 250,000 times more distant than the Sun, vastly farther than anyone would have believed before the invention of the telescope.

Here is a pictorial "zoom out" from the Earth to the distance of Alpha Centauri.

There are 36 stars within 12.5 light years (about 4 parsecs) of the Sun. Click here for a perspective illustration.

The Sun and all these stars, in fact all stars you can see in the sky, are members of the gigantic star system we call our galaxy. It is an enormous disk-like structure about 75,000 light years (450 million billion or 4.5x1017 miles) across. (See the diagram above or click here.) It contains about 100 billion (1011) stars.

The Sun is a perfectly ordinary star. It does not stand out among the myriad of stars in our galaxy.

The Sun is not located near the center of our galaxy --- it is very much in the "suburbs." See the drawing above.

Recall our scale model from the last lecture: if stars near the Sun are modeled as oranges, the oranges would be separated by distances of over 1000 miles! The density of matter near us in the galaxy is very low.

In that scale model, the center of our galaxy would be 10,000,000 miles away (40 times farther than the Moon).

From the Sun's vantage point, we see the disk-like distribution of stars in our galaxy projected on the sky at night as a faint band of light---which we call the "Milky Way".

This interpretation was first proposed by Thomas Wright in 1750. See Wright's drawing here. Wright also conjectured that some of the faint, diffuse patches of light found by telescopes were distant galaxies, something that would not be proved until 170 years later.

### Other Galaxies

It was not until the 1920's that astronomers realized ours is only one among a vast number of galaxies. There are many other galaxies near ours in space. Here is a chart of the galaxies clustered near our own, the so-called "Local Group." Galaxies can be tremendously bright systems intrinsically, and therefore we can see them across enormous gulfs of space. Even with the naked eye, you could see four galaxies. (But these are the only things you can see which are not in our galaxy.)

The Andromeda Galaxy in a long exposure image.
Note: the white dots are all foreground stars in our galaxy.
Andromeda is far beyond them.

The most distant object you can see without a telescope is the Andromeda Galaxy, the most luminous member of the Local Group. The Andromeda Galaxy is comparable in size to our own Galaxy. It is visible as a faint, elongated patch of light on a dark, clear night. Here is a sky map showing how to find it.

The Andromeda Galaxy is 2.5 million light years away, or 15 billion billion (15 x 1018) miles.

## D. The Lookback Effect

"What seest thou else in the dark backward and abysm of time?"
--- from "The Tempest," Shakespeare

The fact that we can detect cosmic objects at such enormous distances has one tremendously important consequence. Light rays from distant stars or galaxies have been traveling for long periods of time before they reach us: in fact, they have traveled one year of time for every light year of distance. Therefore:

BY LOOKING OUT IN SPACE WE ARE LOOKING BACK IN TIME.

Because of this "lookback effect," we are able to see other parts of the universe as they were at earlier times.

For instance, the light you could see tonight coming from the Andromeda Galaxy left its stars 2.5 million years ago, before the modern human species even existed!

This animation shows how light propagates through the expanding universe.

Astronomy is unique in this regard. In no other human endeavor are we actually able to see the past. In effect, astronomers have a kind of time machine at their disposal. They are able to directly study the evolution of the universe as it happened.

Of course, there's a catch:

• The distances to viewable parts of the "early universe" (over several billion light years away) are so large that only very bright objects can be detected, even by our largest telescopes. We must learn to compensate for the resulting biases.

• Also, we can't usefully explore our own personal past in this way. We do see nearby objects as they were in the past, but the lookback is only on the order of nanoseconds.

 The Hubble Space Telescope in orbit Distant galaxies seen in an extract from the "ERS GOODS" deep field survey, obtained with the HST in 2009. Click for a panorama of the full survey (6.8 MB).

## E. The Deep Universe

The universe is filled with galaxies, both smaller and larger than our own. As in the case of the Earth and the Sun, we have found that there is nothing special about the physical properties or location of our galaxy.

Here is a supercomputer simulation of a trip outside our Galaxy to the distance of the largest concentration of galaxies in the nearby universe (the Virgo Cluster), about 50 million light years away (38 MB mpeg file from the National Center for Supercomputing Applications).

Here is another nice simulation of a trip from Earth to the farthest observable reaches of the universe, far beyond the Virgo Cluster. (American Museum of Natural History). Both this and the previous video are based on actual astronomical data sets.

The depths of the universe are plumbed by instruments like the Hubble Space Telescope, our premier orbiting observatory, and the many huge ground-based telescopes built over the last 15 years.

The Hubble Space Telescope has been used to make a number of deep imaging surveys of the distant universe. Click on the image above right for a panorama of one of these. The picture at the top of the page is an extract from the "Hubble Ultra Deep Field," a super-long exposure (over 600 hours) that contains images of the faintest objects ever detected. Click on the image to see the entire Hubble Ultra Deep Field.

This image represents the present edge of the observable universe.

The faintest images here are 10 billion times fainter than you can see with your unaided eye. There are only a few stars in this picture. Everything else you can see is a galaxy, about 10,000 of them in the whole HUDF field.

The objects visible in the HUDF are so distant their light has taken billions of years to reach us. Some of these galaxies are seen as they were over 13 billion years ago!

One of the basic conclusions from studying these distant objects is that they are different from local galaxies in many ways. For instance, the distorted shapes you can see in the picture at the top of the page are rare among local galaxies. In other words, the deep images provide direct evidence that the universe has evolved with time.

The radius of the "observable universe" is given by the distance a light ray can travel in the time since the origin of the universe (i.e. the "Big Bang"). That is about 13.7 billion light years. We expect that more distant objects exist, but light rays from them have not had time to reach us yet.

The total number of galaxies within the observable universe is of order 200 billion. On average, each of these contains about 100 billion stars. So the total number of stars in the observable universe is of order 2x1022.

This is a fascinating number. On one hand, it is amazing that we can calculate it at all and have some confidence that it's correct to within a couple of orders of magnitude. On the other, it is so huge that it gives a good sense of the scale of the observable universe. Needless to say, most of those stars are not detectable individually.

## F. The Infinite Universe

The structure of space and time on the largest scales is governed by Einstein's theory of General Relativity. By combining that with the plethora of recent data about the expansion of the universe and the structure of the most distant regions we can measure, astronomers have concluded that the universe is spatially infinite in volume.

Infinity may be the most difficult concept humans have ever grappled with because it is completely alien to our everyday experience, which, of course, transpires in a finite world. It is impossible to visualize. The ancient Greeks and Indians had discovered the concept of infinity in mathematics, and a number of their thinkers were comfortable with the notion of an infinite universe containing an infinite number of possibly inhabited worlds. Other scientists have been horrified by the implications of an infinite universe.

Consider just a couple of the implications:

The volume of the observable universe is unimaginably large. Yet it constitutes exactly zero percent of the volume of the whole universe.

In an infinite universe, anything that is possible according to the laws of physics must happen somewhere even if it is extremely improbable. That means there is another ASTR 1210 class out there that is exactly like this one. And there is not just one of these but an infinite number of them!

If you try hard enough to explore examples like this, you may get a faint glimmer of understanding of what infinity means---and you will almost certainly find this disturbing. Blaise Pascal (d. 1662), a leading mathematician and physicist, famously recoiled from the concept of an infinite universe:

"...engulfed in the infinite immensity of spaces whereof I know nothing,
and which know nothing of me, I am terrified.
The eternal silence of these infinite spaces fills me with dread."

Star-forming region in a nearby galaxy

## G. Earth in the Context of Cosmic History: The "Top Ten"

We now think we have a good understanding of the broad outline of cosmic history. I list the "top-ten" elements of that outline below, roughly in order of their sequence in cosmic time. Some were already highlighted in Guide 01. For a narrative description of the history of the universe, click here.

1. The universe began about 14 billion years ago in an ultrahot and ultradense state called the "Big Bang" and has been expanding ever since. The spatial volume of the universe is now, and has always been, infinite.

2. Physical structure in the present-day universe originated in tiny irregularities in the distribution of matter during the Big Bang which have been "amplified" over the intervening time by the expansion of space and the force of gravity.

3. The easily observable matter in the universe is organized into galaxies, huge star systems with typical sizes of 10's of thousands of light years containing billions of stars. Our galaxy is at least 12 billion years old. But it is not special in any way.

4. Stars form continuously out of the diffuse "interstellar" gas in our own and other galaxies. The star formation rate was very fast at earlier times but is much more modest now. Some galaxies are quiescent now; ours forms stars at a rate of about 1 solar mass per year.

Sun (in the H-alpha atomic emission line) showing
active regions and a flame-like "prominence."

5. The Sun is a star, with average properties

"Average" means that the Sun is not distinguished from billions of other stars in our Galaxy. This recognition resolves thousands of years of religious, philosophical, and scientific debate.

This is one of the most important discoveries in astronomy. However, it cannot be credited to a single individual, because it involved a long chain of incrementally improving evidence and speculation by many astronomers since the time of Copernicus. The case was clinched by comparative spectroscopy (see Study Guide 10) of the Sun and typical stars late in the 19th century.

"Across the sea of space, the stars are other suns."
--- Christiaan Huygens (1692)

6. Stars generate their energy by burning hydrogen in nuclear fusion reactions. The hydrogen supply is large but nonetheless finite, so this implies that stars must evolve as they begin to run out of fuel. The Sun will eventually burn out. It is middle-aged: it formed about 5 billion years ago, about 60% through the age of our galaxy, and its remaining lifetime is about 5 billion years.

7. Other than hydrogen and helium, the chemical elements are synthesized during fusion reactions in stars. They are recycled outside stars when they lose their outer layers or explode at the end of their lives. All the heavy elements that make up the Earth originated inside stars now long dead.

That is also true of the biologically important elements that constitute all living things. Stars are an essential part of our ultimate human cosmic heritage. They are not merely incidental celestial decoration, as they were often considered in pre-scientific philosophy.

Here's a video featuring Neal Tyson discussing this "most astounding fact."

8. Planetary systems are a normal byproduct of star formation. We now know of over 900 other planetary systems, some including Earth-sized planets. The present estimate is that almost all stars host planets, and most of them probably host Earth-sized planets.

9. Earth is a planet in orbit around the Sun.

It is unique among the presently-known planets for its oxygen-rich atmosphere and surface oceans and for harboring life, which has been present for at least 3 billion years. Most astronomers are confident that there are millions of planets like the Earth in our galaxy, but the extent to which those support advanced lifeforms is debatable.

Human beings are definitely latecomers on Earth: Homo sapiens has been present only for about 200,000 years---just 0.004% of the age of the Earth.

10. Earth's biosphere is highly vulnerable to astronomical phenomena. especially asteroid impacts, solar evolution, magnetically-induced activity on the Sun (because the Earth is inside the Sun's extended atmosphere), and stellar explosions.

Here is a video of magnetically-induced activity on the Sun. It shows vividly how material is flung off the Sun's surface during eruption events.

If you're interested in exploring the astronomical hazards facing the Earth, have a look at Death From the Skies, by UVa PhD Philip Plait (cover shown at right).

Bennett textbook: Ch 1 and Secs 3.4, 3.5.

Study Guide 2

Supplement I (PDF file) Skim and then refer to this later as needed.

Optional: Cosmic History: A Brief Narrative

Optional: browse the material on the structure & evolution of the universe in the Bennett textbook Chs 22 and 23

Optional observing: After you've done the Constellation Quiz (see the next guide) and become familiar with the sky, you might want to go to a good dark location on a clear, Moonless night and try finding:

The Milky Way, the plane of our galaxy seen edge-on. The best views in the evening from the northern hemisphere are in July through October, when it stretches from the northern to the southern horizon. A deep, wide-angle exposure is shown here.

The Andromeda Galaxy, the most distant thing (2.5 million light years) you can see with the naked eye. See the finding chart here. The Andromeda region is visible in the evening sky August through February.

The Scutum Star Cloud. A concentration in the northern Milky Way composed of about 1 billion stars. See the finding chart here. The Scutum region is visible in the evening sky July through October.