PART 1 - OVERVIEW OF THE UNIVERSE

Introduction - why is astronomy interesting?

• There are some intimate connections between astronomy and our existence. A brief history of the universe:
  • About 14 billion years ago, the Universe as we know it came into existence.
  • Amazingly, we know something about its state in its youth. It was a very different place than it is now. Unlike today, when the Universe has lot of empty space, at that time, it was very different: hot and dense and composed of light
  • In the first few minutes, the pure energy was transformed into the basic particles we see today: protons, neutrons, and electrons. However, only the simplest elements were made: hydrogen, helium, and a tiny bit of lithium and beryllium
  • As time passed, the Universe expanded rapidly and cooled off
  • After a few million years, the matter in the universe, which was previously smoothly distributed, collapsed into blobs under the force of gravity. At some point, in the centers of these blobs, stars were formed.
  • The stars and gas around them separated into distinct groups in space which we now call galaxies
  • Each star in each galaxy acted as a nuclear reactor, converting simple elements into more complex ones and producing light in the process
  • Some of the stars exploded, sending the more complex elements back into the gas, from which new stars were formed. Around some of these stars, some of the complex elements condensed into rocky balls which we call planets.
  • About 5 billion years ago, a star was formed in the outer region of one of the galaxies. Around this star, a system of nine planets formed. The size and temperature of the 3rd planet from the star was such that liquid water existed on its surface.
  • On this planet, the combination of elements along with the light from the star allowed chemical reactions to take place which allowed the formation of simple life. The main elements in most life forms included some of the elements that were synthesized in previous generations of stars: carbon, nitrogen, and oxygen.
  • Over several billion years, the life forms changed and evolved. Very recently, only a few million years ago, humans appeared on the planet. They lived simply for millions of years. They probably looked at the stars and wondered about them, not dreaming that their existence, in a direct way, depended on previous generations of stars. Connection 1: Humans are literally made partially of stardust.
  • Astronomy has also been used practically by humans. Very recently (on the astronomical time scale), humans developed agriculture and depended on their observations of the sky to help them determine when to plant crops. Some cultures used observations of the night sky for navigation. In the last several hundred years, astronomy has played a fundamental role in developing basic understanding of several underlying physical laws, in particular, the laws of motion and gravity. Connection 2: Astronomy played a role in the development of modern civilization.
  • Humans are part of astronomy. We send spacecraft from the Earth to other places, and have an effect on the global properties of our own planet, Earth. Connection 3: By our existence, we affecting astronomical objects, at least within the Solar System, and especically, the Earth.

• Recent advances in technological tools make today a special time; astronomy is at a very exciting stage! This technology allows us to see much more of the Universe than we can by eye, and our view of the Universe has changed dramatically. ((pictures in slideshow form))
  • Where Earth was thought to be large and immutable, changing gradually, if at all, we now have seen Earth from space and realized how small and fragile it is in the overall scheme of things. We know that the very continents move around and collide with each other. It's possible that the history of Earth is dramatically influenced by the impact of comets on its surface.
  • Where people once saw the Sun an unchanging, we now know it is quite dynamic, undergoing changes that can influence Earth. We know that the Sun will not shine forever.
  • Where people once saw planets as bright points of light, we now see them as worlds of their own, with rapidly changing atmospheres and systems of moons and rings around them. Some of these worlds have active geology and perhaps even harbor some form of life.
  • Where people once saw unchanging stars and planets, we now see material between stars, newly forming stars, and exploding stars
  • People now see galaxies and can measure their motion through space and understand that the Universe itself is expanding.
  • We realize that the Universe is very dynamic: there are astrophysical phenomena operating on all time scales, from the slow evolution of stars and galaxies on timescales of billions of years, to supernovae and gamma ray bursts on the timescales of days and seconds.
  • New technologies of big telescopes and telescopes in space allow us to look far back into time and see what the Universe looked like long ago.
  • There are lots of recent astronomical developments
    • 2003 Science news of the year: the contents of the Universe (WMAP map of the microwave sky , Sloan Digital Sky Survey , contents of the universe)
    • 2004: A rover on Mars! (Mars globe , Spirit landing site , the scenery!)
    • 2004++: Cassini spacecraft at Saturn, Huygens probe into Titan

  • The development of a fundamental understanding of the universe is still in progress. Just in the past few years, astronomers have revised their ideas about the overall shape, age, and contents of the Universe. In the current picture, large fractions of the Universe are thought to be in some still-unknown forms of ``dark matter'' and ``dark energy''.

  • Although we think we know a lot, there's a good chance we don't. Mankind has been around for only a tiny fraction of the time the Universe has been around, and the technological age has been only a tiny fraction of that. If the history of the universe were placed into the framework of a week, mankind would have appeared about a minute before midnight on the last day of the week, and the technological age just a fraction of a second before midnight.

• Astronomy is a crossroads science, involving geology, physics, chemistry, and mathematics

• Astronomy is trickier than other sciences because, for many objects, we can't visit what we study. However, the vast distances involved give us a unique capability - the ability to look into the past!

• New Mexico is a great place to look at the sky!

What are the goals for this class?

• Come out of this class thinking astronomy and science are as interesting, or more so, than you did at the beginning of the class
• Communicate to you how we have come to understand something about the Universe, and to give you an appreciation of the Universe and of the exceptional ability of humans to learn about it
• Understand that science is a process, not a collection of facts.
  • Questions are more important than answers
  • Try to formulate class around questions, rather than rote recital of facts. See the class outline.
• Understand what a scientific result is, and how it differs from an opinion
• Develop a broad appreciation for science & its impact on society.

What good does studying astronomy do for you?

• Helps you in the process of logical thinking - look critically at what you are told and assess for yourself whether it is reasonable.
• Helps you appreciate the world aesthetically, if you can understand why things appear and behave as they do
• Improve the condition of the world by acting on your knowledge and understanding of the scientific process
• Get some enjoyment out of looking at the sky and imagining some of the vast distances and incredible events that are going on out there
• Satisfies your laboratory science general education requirement!

Class logistics

• See the syllabus.
• How to do well in this class:
  • Come to class. Ask questions if you have them
  • Review the class notes after each class. Even 5-10 minutes will be useful!
  • Do the online homework assignments; note what you get right and what you get wrong, and if you get questions wrong, understand why. Use the supplementary online material.
  • Read the labs before you come to lab. Set aside 30 minutes to write the lab summary. Think about the lab, look up information if needed, then write about it
  • Don't get any zeros. Don't miss handing in labs, taking exams, or doing the online homework.

What is science, and why should we understand and do it? (presentation format)

• There is a distinction between observational data and theories that attempt to explain them.
  • Data are determined from observations. Sometimes, the process of obtaining, or interpreting, data can be tricky. An interesting part of the scientific process is understanding how we know certain things. For example, consider the following facts, and think about how we know them: the Earth is round, the Earth goes around the Sun, the Moon goes around the Earth. How do we know? Science involves formulating critical questions to determine whether ideas are in fact true.
• What is the difference between a theory, a scientific theory, and a law of nature?
  • It is usually very difficult to prove that something is correct. Most of the time, scienctists spend time trying to prove that theories and ``facts'' suggested by other people are wrong! This is a hallmark of a scientific theory
  • In general, a succesful theory not only needs to provide a plausible explanation of a fact, it generally needs to make predictions for new observations that can be used to verify it. Theories are often also judged by their level of simplicity, i.e. a simple theory that explains a series of observations is usually preferred to a more complex one that makes no better or additional predictions.
• In this class, as in science, a fact or theory means something that so far no one has been able to prove wrong; however, people have generally tried very hard to do so!
  • Skepticism is an important part of the scientific process. ``Qualification'' of opinions is a trademark of scientists, but can be misinterpreted by non-scientists.
    • Skepticism, however, should not become a matter of faith. You cannot be so sure that a theory is wrong that you will never accept it, even if no contradictory observations come to light. A theory is accepted until it can be disproven, and the onus is on skeptics to find contradictions with observation.
  • When judging the validity of scientific results, one might consider the information, attempts to invalidate the information, the sources of information, and possible biases of the investigators.
• Much of the ``art'' of science is in coming up with the ideas, or hypotheses, to test. In research, the initial hypotheses are generated by scientists for any of a number of reasons. In astronomy, many hypotheses are generated by looking at pictures and trying to figure out what might explain the way things look.
  • Group exercise - pictures
• We will try to concentrate on this class on understanding not just what we know about astronomy, but on how we know it, and also, on what we don't know; at an introductory level, this can be challenging, however.

What is the difference between astronomy and astrology?

• Class query:
  • How many people know the difference between astronomy and astrology?
  • How many people believe in astrology?
  • How many people read horoscopes?
• Astrology is something which purports that the position of the planets and the stars at the time of your birth determines the course of your future life.

• Is astrology a science? There is nothing about this hypothesis which is directly non-scientific; scientists are free to come up with whatever crazy theories they want. However, the process of science is asking questions about hypotheses, so any good scientist will ask the questions: does the hypothesis of astrology work? Is there any known physical mechanism by which we expect astrology to work?

• There is no physical mechanism for astrology to work. It turns out that all of the phenomena that we see in the universe around us can be explained with only four basic forces: gravity, electromagnetism, and two nuclear forces known as the strong and weak forces. Via these forces, the location of planets and stars at the moment of birth are not relevant, and besides the whole idea is a bit strange since we know that humans develop over a nine month period before they are born.

• There is no evidence that astrology actually works. Note that the predictions of astrology may work sometimes; almost certainly, some of these predictions will work sometimes by chance! But for astrology to be a legitimate science, it has to work all of the time. Gravity doesn't just work ''sometimes''; it's the law! Here is a link to some studies on the predictions of astrology.

• Certainly, the widespread practice of astrology, namely horoscopes, is incredibly suspect. One only needs to look at the different horoscopes for a given day predicted by a variety of astrologers!

• Does astrology have the right to be respected as a religion? If so, it is a fairly shallow religion with no moral guidelines, as it believes things are predetermined.

• Astrology is an examle of what is known as a pseudo-science, or a superstition. Is it ok to regard pseudo-sciences are harmless entertainment? Maybe, depends how far you take them. When elected officials consult astrologers, I think there is some cause for concern.

• This is a real issue. As the world becomes increasingly technologically sophisticated, it seems that more people are looking for ``simplicity'', and have difficulties assessing the validity of the different things they are told. However, there is a real and fundamental difference between a science and a pseudo-science. This lies in the degree of scrutiny to which hypotheses are subjected. In sciences, people try hard to disprove theories.

• This is not to say, however, that even scientific results cannot be manipulated. It is a good skill to be critical!

What is astronomy and what do astronomers do?

• Astronomy is the science of studying objects seen in the sky. Astronomers try to understand the nature of astronomical objects, figuring out what is out there, details about the physical nature of the objects, and try to develop an understanding of why the objects look what they look like, how they got there, and how they might change with time.

• Jobs: astronomy doesn't have too many immediate applications which provide profit, so many astronomers are hired by the public sector: universities and government research centers. However, increasingly, astronomical missions are being built by the private sector, so there's a growing number of astronomers hired by aerospace industry. Astronomy/physics training, however, seems to be in moderately high demand, for a variety of apparently unrelated fields, e.g. on Wall Street!

• Research. Generally, there are three main types of research activity that astronomers might do.
  • observational research: collection and analyze data, namely light, from astronomical objects. Several stages:
    • Collection of light usually requires staying up at night at a telescope. However, in this day and age, the analysis of the data takes a lot longer than the collection, so even purely observational astronomers don't observe that many nights; typically, an astronomer might spend a few to a dozen nights per year at the telescope. These days, most information is recorded electronically, and observing at the telescope usually means sitting in a warm room controlling the telescope and instrument with a computer, looking at the data as it comes in to judge whether more data is needed, listening to music, eating cookies, etc.

    • The process of data reduction comes next; in this process, astronomers correct for all of the features that are introduced by the telescope and the Earth's atmosphere.

    • Astronomers try to interpret what they are seeing in the light of some model of what is going on in their objects, or try to come up with some model of what is going on which other scientists can try to shoot down! Computers usually play a large role in astronomical research these days, as single pictures consist of LOTS of individual pieces of information.

  • theoretical research, where astronomers try to predict what objects in the sky will look like based on the laws of physics. This may involve lots of analytical work or computer modelling.

  • Instrumentation, where tools are developed for use at the telescope.

• Many astronomers teach, and do public outreach

• There are some direct applications of astronomy: space mission support, global positioning, etc., etc.

What is going on with astronomy at NMSU?

• We have an astronomy department which consists of 8 faculty members plus some research faculty. We offer a graduate PhD program, but we don't offer an undergraduate major, partly because we feel that astronomers should have a firm basis, e.g. an undergraduate degree, in physics, and partly because we wouldn't get enough majors! We have about 20 graduate students.

• NMSU operates a large observatory at Apache Point, about 2 hours of Las Cruces. There are several telescopes at this site. The largest is 3.5m diameter, and is shared with University of Washington, Univ. of Chicago, Princeton, Washington State, and Johns Hopkins; NMSU gets about 15% of the time. We also have a 1m telescope. There is also a 2.5m telescope that is doing an a survey of all objects in a big piece of the sky; NMSU is a partner in this project.

• Apache Point is ``next door'' to the National Solar Observatory in Sunspot NM. There is a joint visitor's center in Sunspot.

• In the department at NMSU, we do a wide variety of research: solar system, stars, galaxies, observation and theory.

• What do I do?
  • Looking for supernovae with the Sloan Digital Sky Survey telescope to try to learn more about the existence, and nature of, dark energy
  • Use APO 3.5m to study detailed spectral properties of nearby stars to learn about what they are made of
  • Observations with the Hubble Space Telescope and ground observations
  • Stellar populations in nearby galaxies
  • Spiral galaxy structure
  • Globular clusters in galaxies

Overview of objects in the Universe

The Universe is LARGE, mostly EMPTY, DYNAMIC, and OLD

The Solar System

• The Solar System is the collection of objects which are associated with the Sun by gravity.

• Contents of the Solar System and their properties:
  • Sun. Biggest object in solar system by far and contains most of the mass (> 99 %). Shines with energy produced by nuclear reactions.

  • Objects that go around the Sun
    • Planets. Objects in roughly circular orbits, with roughly spherical shapes (but a precise definition is very difficult!). From a historical perspective, there are nine (but stay tuned!): in order from closest mean distance from the Sun to furthest, they are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. All of the orbits are in approximately the same plane. We can see planets because they reflect sunlight; in addition, as we will see later, planets also glow because they are warmed by the Sun, but this glow is mostly in a kind of light that our eyes don't see called infrared light. Masses, sizes, and compositions vary. There are two general classes of planets, the inner, rocky planets, and the outer, gaseous planets. Pluto doesn't fit nicely in either category.

    • Asteroids: smaller rocky (usually) non-spherical bodies which orbit the Sun. Most, but certainly not all, orbit between Mars and Jupiter in the asteroid belt.

      $\bullet$ Asteroids range in size from a few hundred miles down to very small particles. The latter may also be called meteoroids.

      $\bullet$ When meteoroids collide with earth, they burn up in the Earth's atmosphere, creating meteors, or shooting stars. Occasionally, small pieces make it down to the surface of the Earth; these are called meteorites.

    • Comets: smaller icy bodies which orbit the Sun.

      $\bullet$ Comets are often in very elongated orbits around the Sun. They spend most of the time far from the Sun, but when they come near the Sun, material evaporates (sublimates) from their surface, and it is pushed away from the comet by pressure coming from the Sun. This causes comets to develop bright tails which can be seen from Earth when a comet comes close to the Sun.

      $\bullet$ Appears that there are two families of comets: Kuiper belt and Oort cloud.

    • Recent discoveries of new ``planets'' and other objects:

      $\bullet$ Eris (2003UB13) discovery, size, and orbit (reference website)

      $\bullet$ Sedna (reference website)

      $\bullet$ Kuiper belt objects and the status of Pluto: In past decade or so, astronomers have been detected increasing numbers of objects that at distances from the Sun equal to that of Neptune or even greater, and these are known as Kuiper belt objects. Some of these objects are relatively large, and this has raised the issue of whether they should be called planets or not; currently they are being called ``dwarf planets''. In fact, many of these Kuiper belt objects have properties similar to that of Pluto (ice-rock composition), so it is likely that there is really a class of objects orbiting the Sun at large distances, of which Pluto is the brightest (but not the largest!). Perhaps it's just a bit of semantics about what should be called a planet and what a Kuiper belt object.

    • What is a planet? Cultural definitions vs. ``scientific definitions''. Does it matter?

  • Moons or satellites. These orbit around planets. Earth, Mars, Jupiter (4 big ones: Io, Europa, Ganymede, Callisto), Saturn (1 big one: Titan) (Titan atmosphere!), Uranus, Neptune (1 big one: Triton), and Pluto all have moons. Some moons are spherical, others are not. Some the moons of the outer planets are nearly as big as some of the inner planets! Our Moon is somewhat unusual in that is it rather large compared to the size of the planet (Earth) around which it orbits; Pluto also has a relatively large moon.

  • Sizes in the solar system:
    • The Sun is by far the largest object in the solar system. If the Sun is represented by a circle 1 meter in diameter, than the Earth is just 0.9 cm in diameter! Jupiter is the biggest planet, at 10 cm in our model, and Pluto the smallest, at 1.6mm! (Relative size of planets)).

    • Even more striking, however, is the amount of empty space in the Solar System. The objects in the Solar System are tiny compared to the distances between them.

    • Group exercise In our model, the distance between the Earth and the Sun is 107 m (distance to the sundial from our classroom), and the distance to Pluto is 4.3km (the mall!). (Sun is 1.4 million kilometers in diameter. The distance from the Sun to Earth is about 150 million kilometers, i.e., about 100 times the diameter of the Sun. The distance from the Sun to Pluto is about 6 billion kilometers, or 40 times the distance from the Sun to Earth.)

    • Most of the Solar System is empty space!

  • Motions in the Solar System
    • Objects revolve around the Sun, because of gravity. The shape of all orbits is a shape called an ellipse, which we'll talk more about later; however, the shape of planetary orbits are close to circular. All planets revolve in the same direction. All of the planets revolve in approximately the same plane as well.

    • Objects rotate around their own axes. Most rotate in the same direction as the direction of revolution, but there are exceptions (Venus, Uranus).

    • The whole solar system moves through space.

  • We've gone through a quick survey of what is in the Solar System. However, remember that the science part of this consists of other things as well:
    • How did people come to figure out that this is what the Solar System is like? How do we know where all of these objects are, how big they are, and what they are made of?
    • Why is stuff in the Solar System the way that it is? The reason that the global properties of objects in the Solar System are the way they are is likely related to the way the Solar System formed. Whatever theories we have about how the Solar System formed must explain some or all of the observed facts about the Solar System: the revolution of the planets in the same direction and plane, the difference between the inner and outer planets, why planets are round, etc. In fact, there is a reasonbly well developed theory for Solar System formation which we'll discuss later after we have some more background in important physical processes. But there are still many outstanding questions...

The Milky Way Galaxy

• Our Sun is one of billions of stars which are held together by gravity in what is known as the Milky Way galaxy. We now know that at least some other stars have planets orbiting them, so it is likely that there are many other ``solar systems'' out there.

• The distance between stars is much larger than the size of stars, or even the size of the Solar System. On our scale model where the Sun is 1m in diameter, the nearest star would be halfway around the Earth! As in the Solar System, most of the galaxy is empty space.

• All stars are not the same.
  • They come in different masses, with many more low mass stars than high mass stars. Our Sun is of intermediate mass. Mass measures the amount of matter in a star.
  • Stars also come in different sizes, giving rise to the terminology of giant stars and dwarf stars.
  • Stars also come in different temperatures, which is related to the color that they appear.

• Stars evolve: they are born, live their lives and die. For high mass stars, lifetimes are millions of years, for low mass stars, billions of years.

• Inside our galaxy, stars come as both individuals, in small groups (binary stars, triple stars, etc.) and in clusters. Star clusters come in two types, open and globular clusters. Clusters have turned out to be very important for astronomers to understand how stars work, because stars in a cluster all appear to be the same age, but in a variety of masses. Binary stars are very important as they are used to measure the masses of stars.

• Stars in our galaxy are not distributed uniformly.
  • Most stars are found in a relatively thin disk of stars, which has many more stars in its central regions than in its outer regions. The Sun is located about 2/3 of the way out in this disk. Looking through this disk from our location in the Milky Way is what causes the appearance of the Milky Way as a band around the sky.

  • A minority of stars is found in a roughly spherical halo around the disk. For some reason, globular clusters are found spread throughout the halo, while open clusters are only found in the disk.
  • Here is a diagram of the shape of the Milky Way and of the location of objects within it. Here is an actual composite picture in infrared light

• There is stuff between stars which is called interstellar matter: it is composed of gas and dust

• There is a ongoing relation between the stars and the interstellar matter in galaxies. New stars are formed out of interstellar matter, and old stars eject much (or all) of their mass back into the interstellar matter when they die.

• Inside our galaxy, both stars and the interstellar matter move. In the disk, all stars orbit the center in the same direction in roughly circular orbits, although it takes a few hundred million years for the Sun to orbit the galaxy once. In the halo, stars do not all appear to rotate in the same direction, and orbits can be highly elongated. As with motions in the Solar System, these observations provide clues about how the galaxy may have formed.

• As we discussed for the Solar System, some of the most interesting stuff about this information we've given about the Milky Way galaxy is an understanding of how we figure this stuff out, and how the Milky Way got to be the way it is. Any theory of how our Galaxy formed must account for the things we observe about it: it's shape, motions, etc. Theories for galaxy formation are still in the process of being developed. Outstanding questions:.....

Galaxies

• Our galaxy is just one of billions of different galaxies. A galaxy is a collection of gas and stars held together by gravity. All of the stars we see in the sky are in our own galaxy, mostly relatively nearby to the Sun.

• Galaxies come in a variety of shapes. Galaxies are generally classified in one of several categories:
  • Spiral galaxies have stars concentrated in a disk (which can either be seen face-on or edge-on). Some stars in the disk are concentrated in a spiral pattern, from which this category gets its name. Spirals have stars as well as a significant amount of interstellar matter.

  • Elliptical galaxies have stars which are found in an elliptical ball. Elliptical galaxies can be nearly spherical or they can be elongated. Elliptical usually have less interstellar matter than spirals, with very little dust.

  • Irregular galaxies don't fit into either category.

• The Milky Way galaxy is a spiral galaxy. (How do we know this? Why does the Milky Way have its name?)

• Galaxies are very large. The distance between stars is much larger than the size of individual stars.

• Motion inside galaxies:
  • In spiral galaxies, both the stars and the gas move in roughly circular orbits.
  • In elliptical galaxies, the stars move, but in more irregularly shaped orbits.

• Outstanding questions about galaxies: why are there several different types of galaxies? Is there something different about how different galaxies form? Or something different about what happens to galaxies after they've formed?

The Universe of Galaxies

• Galaxies are not spread uniformly through space. They come in clumps called galaxy groups and galaxy clusters. The largest clusters can have thousands of galaxies, which are held together by gravity.

• We live in a small group of galaxies, composed of about 20 members, called the Local Group. It is a few million light years across. There are two big galaxies in this group (the Milky Way and Andromeda), several intermediate sized galaxies, and a bunch of small faint galaxies. The closest galaxies to us are called the Magellanic Clouds (the Large Magellanic Cloud and the Small Magellanic Cloud; they can be easily seen, but only from the southern hemisphere.

• The nearest cluster of galaxies is the Virgo cluster, several tens of millions of light years away.

• On the largest scales, galaxies appear to be distributed in filamentary-like structures. Many galaxy clusters and groups are often found in proximity to each other along filaments, giving rise to what are galaxy galaxy superclusters, but superclusters are not isolated from each other in space in the way that galaxies are.

• Galaxies move and sometimes collide with each other.

• While galaxies do move relative to each other, there is an additional apparent motion: the entire universe is expanding.
  • Essentially all galaxies we see are moving away from us.

  • The speed at which they are moving away is proportional to their distance; more distant galaxies appear to be moving away faster than closer galaxies. In this situation, no matter what galaxy we lived in, we would see that all other galaxies would be moving away from us.

  • Consequently, we believe the entire Universe is expanding, most likely without any center of the expansion.
    • Even though the Universe as a whole is expanding, small regions of the Universe where there are objects (groups or clusters of galaxies) can be contracting under the force of gravity

  • Given the expansion rate, we can estimate how long ago the Universe would have been at a single point, and we find that this occurred 10 to 20 billion years ago, assuming that the universe has always been expanding.

  • The theory that the Universe was originally much more concentrated and that there was a time when everything was collapsed together is called the Big Bang theory
    • The Big Bang theory was motivated by the observation that all galaxies appear to be moving apart from each other. However, this observation alone leaves open the question about whether the Universe has always been expanding.

    • The Big Bang theory got a major boost with the discovery several decades ago of the microwave background radiation; the entire sky is glowing very faintly in a kind of light called microwave light. This was predicted as a result of the Universe having been hotter when it was denser; in the microwave background, we are seeing the glow of the hot universe long ago.

    • Within the context of the Big Bang theory, galaxies are thought to arise from originally very small inhomogeneities that grow by the force of gravity.

    • This picture has received major support from observations in the past decade by the COBE and WMAP satellities, which observe very small ``lumpiness'' in the microwave background.

  • What is the fate of the Universe?
    • The expansion of the Universe is well understood in the context of today's most sophisticated theory of gravity, which is called general relativity. The equations of general relativity allow for the possibility that the Universe will continually expand and also for the possibility that it will eventually contract.

    • The critical item that determines what will happen is the density of matter in the Universe: if there is enough matter, its gravity can evenutally cause the Universe to recontract

    • Many recent observations, especially those made by the WMAP satellite, strongly indicate that there is not enough matter to cause the Universe to recollapse, in other words, that it will continue to expand forever. These observations also help to pinpoint the age of the Universe (how long ago the Big Bang was) at 13.8 billion years.

    • Recent observations, surprisingly, suggest that the expansion rate is not slowing down as one would expect. This has led to the idea that there is something in the Universe that is causing it to accelerate. We don't know what this is, astronomers have dubbed it ``dark energy''
  • What is the shape of the Universe?
    • The amount of both matter and energy in the Universe also affects the ``shape'' of the Universe.

    • A useful analogy to describe the shape of the Universe can be found in two dimensions when one considers the properties of a flat plane vs. those of the surface of a sphere (or, separately, a saddle shape). On these different surfaces, for example, the behavior of parallel lines or the sum of the angles of a triangle are different. If there is not enough matter and energy, the Universe will be curved in an ``open'' sense, if there is just the right amount, it will be flat, and if there is more than this, it will be curved in a ``closed'' sense. The shape of the Universe is related to its extent; an open universe has infinite size, while a closed universe has a finite size.

    • Recent observations suggest that the shape of the Universe is ``flat'', i.e. the analog of a flat plane in three dimensions. However, the amount of matter in the Universe does not appear to be sufficient to make it flat, hence the recent observations suggest that there is a large component of ``dark energy'' in the Universe that in fact dominates the total matter-energy density and makes the Universe flat. People are still trying to figure out what this dark energy might actually be!

    • Note that this dark energy is not to be confused with ``dark matter'', which also exists and which we will discuss in a few weeks!

Distances in astronomy - how do we know how far away things are?

• We are used to measuring distances in two main ways:
  • Direct measurement, e.g., with a ruler
  • Measurement using geometry/trigonometry, e.g., surveying

• Distances is astronomy can be tricky to measure, especially for very distant objects, and determining distances to the most distant objects involves several steps.
  • Direct measurements are not possible, although a form of them using light travel time (laser ranging), can be used for some objects within the solar system.

  • For nearby astronomical objects, we can measure distances through a geometric technique known as parallax, which is the same technique used by surveyors on Earth. This involves observing an object from two different vantage points, and measuring how the direction we need to look changes from one vantage point to another; the farther apart the vantage points, the more the direction will appear to change. The amount the direction changes gets smaller as objects get farther away.

  • For more distant objects, we look for objects which appear similar to those at closer distances, and make the assumption (which can often be closely tested) that they have the same intrinsic size or brightness as closer objects. The apparent size or brightness of objects depends on the distance: more distant objects appear smaller and fainter than identical nearby object.
    • Measuring the apparent size of an object gives its distance if we know its true size. The apparent size will be smaller if the object if farther away. The relationship is simple:
      apparent size $$\propto$$ $${\mathrm{true size} \over \mathrm{distance}}$$
      If we can measure the apparent size, and know what the true size is, we can determine its distance. (Demo using a ruler and a yardstick, how do apparent lengths change?)

    • Measuring the apparent brightness of an object gives its distance if we know its true brightness. The apparent brightness of an object is fainter if it is farther away.
      apparent brightness $$\propto$$ $${\mathrm{true brightness} \over \mathrm{distance}^2}$$

      If we can measure the apparent brightness, and know what the true brightness is, we can determine its distance. (Demo using lightbulbs, how brightness changes with distance, how you can have lightbulbs of different intrinsic brightness, and how you might recognize the difference, e.g. lightbulbs with different colors).

    • Some examples of objects used as standard candles, that is, objects with known intrinsic brightness are: a kind of variable star called a Cepheid variable, and a particular kind of supernova explosion.

  • By the time we talk about measuring distances to the most distant objects, we are depending on several previous measurements of distances to closer objects. As a result, distances become more and more uncertain as they become larger. Still, it is quite amazing the we believe that we know distances to astronomical objects, even those at the largest distances, to a reasonable level of accuracy: the distances we measure may be off by a few percent, or even a few tens of percent for the most distant objects, but they are not off by much more than this!

• Since distances in astronomy are so large, they're difficult to comprehend if we use normal measures of distance such as the mile or kilometer. Astronomers often use different units to measure distances so the numbers are not so gigantic.
  • Inside the Solar System, differences are often measured in a unit called the astronomical unit. One astronomical unit is defined as the average distance between the Earth and the Sun. Pluto is about 40 astronomical units away from the Sun.

  • For larger distances, astronomers often use a unit called the light year, which is another measure of distance. A light year is the distance than light travels in one year; it is a long way, as light travels at 186,000 miles per second! The Sun is about 7 light minutes away, Pluto is several light hours away, the nearest star is several light years away, the Galaxy is thousands of light years across, and other galaxies are millions to billions of light years away.

  • Astronomers also use a distance unit called a parsec, or a kiloparsec (1000 parsecs); we won't go into the details here, but a parsec is about 3.25 light years. The units of parsecs come directly from the geometric technique of parallax.

• Review of the objects in the Universe, with rough size scales
  • asteroids
  • comets
  • dwarf planets
  • inner planets
  • outer planets
  • Sun
  • solar system: light-hours
  • nearby stars: light-years
  • center of the Milky Way: tens of thousands of light years
  • Milky way galaxy: hundred thousands of light years
  • nearest galaxes: millions of light years
  • distant galaxies: billions of light years

• Since we see objects so far away, we are seeing what they looked like in the past, and this demonstrates that the Universe has been around for a long time.