Wednesday, May 27, 2009

Hubble's Law Proves the Big Bang

What is the meaning of Hubble’s Law, which says that all the galaxies in the universe are moving away from Earth at velocities that are proportional to their distances from Earth?

Hubble’s Law leads us directly to conclude that the universe is expanding and that it had a beginning – the Big Bang!

To understand this, consider a simpler, down-to-Earth example. Imagine that we were on the top of the Seattle Space Needle (on a clear day in Seattle) using a telescope to look at traffic leaving the city for a long weekend. Also imagine that we discover that every vehicle on the road happens to be moving away from us at a velocity proportional to their distance (Hubble’s Law v = H d). Thus if we see a minivan 75 miles away going 25 mph, we know H = 1/3. And if we see a sports car 150 miles away, it must be going 50 mph per Hubble’s Law. If none of the vehicles changed their velocities, then where were they 3 hours earlier? Since the minivan covers 75 miles in 3 hours (25 mph times 3 hours), and it is 75 miles away now, it must have been at the base of the Space Needle 3 hours ago. Similarly, the sports car covers 150 miles in 3 hours, so it too was at the Space Needle 3 hours ago. In fact, if every vehicle on the road is obeying Hubble’s Law, then every one of them must have been in the same place 3 hours ago (probably when everyone got off work). This must be true because we can rearrange Hubble’s Law to read d – v/H = 0.

What’s all that got to do with the universe? If all the galaxies are moving away from us at velocities proportional to their distances (Hubble’s Law), as we observe that they do indeed, then all the galaxies must have been in one place at one time long ago – we call that moment the Big Bang.


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Sunday, May 24, 2009

Hubble's Constant and the Expansion of the Universe

We discussed earlier how Henrietta Leavitt discovered how to use Cepheid variable stars as one of astronomy’s most precise “standard candles” to measure vast distances. Edwin Hubble built on Leavitt’s discovery to make some of the most important scientific advances of the 20th century.

Before Hubble’s work, astronomers thought our galaxy, the Milky Way, was the entire universe – they really didn’t have a concept of galaxies being separate structures within a larger universe.

Starting in 1919, Hubble began measuring the period and apparent brightness of numerous Cepheid variables in various so-called “nebulae” that were then thought to be fuzzy patches with “lots” of stars. In 1925, Hubble announced his findings. Using Leavitt’s work, he found that some of these “nebulae” were immensely farther from us than the rest of the Milky Way. And to be as bright as they appear to us, they must have billions of stars – they must themselves be separate galaxies, some comparable or even larger than our galaxy. Almost overnight, our concept of the size of the universe was literally blown away – its vastness was far beyond anyone’s wildest dreams.

In 1929, Hubble went even further. He combined his galaxy distance measurements with “redshift” measurements of the same galaxies. Redshifts are changes in the frequency of starlight that allow us to determine a star’s (or a galaxy’s) velocity. What Hubble discovered was that almost every galaxy was moving away from us, and for distant galaxies, their recessional velocity was proportional to their distance from Earth. In math, this is written v = H d, where d is the galaxy’s distance away, v is its recessional velocity, and H is a number we call the Hubble “constant.”

Based on Hubble Space Telescope data published by NASA two weeks ago, we now know H = 51,000 mph per million light-years, to a precision of 5%. (A light-year is the distance light travels in one year, about 6 trillion miles.) This means a galaxy 10 million light-years away is moving away from us at 510,000 mph, and a galaxy 100 million light-years away is moving 5.1 million mph.

The value of H has an enormous impact on the evolution of the universe. For nearly a century, measuring H has been one of the most important and most difficult tasks in astronomy and the source of great controversy. Edwin Hubble’s first measurements were about 7 times too high and had a very large measurement uncertainty. Even as recently as the 1990’s, some astronomers believed H was twice as large as what others thought. Using the space telescope named in his honor, Hubble’s constant has been measured with great precision.

Sunday, May 17, 2009

More on Hubble

Edwin’s Hubble’s famous discoveries about our universe were made possible by the remarkable work of Henrietta Leavitt. Overcoming her deafness and male chauvinism, Leavitt became the first famous American woman astronomer by solving one of astronomy’s greatest challenges.

After graduating from Radcliffe, Leavitt joined the Harvard Observatory in 1893 as a low-level technical assistant – women were not allowed to operate telescopes at that time. Her job was to count stars on photographic plates and measure their brightness.

Leavitt saw beyond her mundane task and discovered a special relationship for stars of a certain type — Cepheid variables. These are stars whose brightness waxes and wanes in a regular periodic fashion. Several types of stars vary in brightness, but Leavitt relaized that among Cepheid variables, the longer it took the star to go through its brightness cycle the brighter the star was at its peak. In fact, Leavitt demonstrated that from the length of a Cepheids’ cycle, its “period”, the star’s true, or “intrinsic”, brightness can be determined with great precision. Then by measuring how bright that star appears to us on Earth, its distance from Earth can be calculated. Leavitt discovered how Cepheids could become one of astronomy’s most precise “standard candles.”

As the notion of standard candles is of great importance in astronomy, it merits a bit more explanation. Imagine looking down a residential street at night and seeing a row of porch lights. If we knew that each house had a 100 watt blub on its porch, and we knew the distance to the nearest house, we could figure out the distance to every one of the other houses. We would measure how bright each porch light looks from our house and use the fact that the intensity of light drops with the square of the distance it travels – the light from a house twice as far away will appear one-quarter as bright. Astronomers can identify Cepheids, measure their periods, and, using Leavitt discovery, compute their distance.

This was huge! Measuring the distance to very remote objects has always been the most challenging task in astronomy. Thanks to Henrietta Leavitt, astronomers were finally able to measure vast distances – distances on a galactic scale and beyond.

With Leavitt’s discovery, Hubble forever changed our view of the universe. He recognized the importance of Leaviit's contribution and recommended that she be awarded the Nobel Prize in Physics. Unfortunately, Leavitt died in 1921, before the completion of Hubble’s work, and Nobel Prizes are not awarded posthumously.

Monday, May 11, 2009

Launch of Hubble Space Telescope Repair Mission

On Monday, May 11th, NASA launched the fifth and final repair mission for the Hubble Space Telescope (HST). Originally launched in April 1990, HST has provided some of the greatest advances in our understanding of the universe. The repair mission will replace failed gyroscopes, batteries and instruments, extending the life of the world’s most important telescope to 2014, and perhaps beyond.

The chart illustrates just how powerful the HST truly is. The chart shows how the amount of detail that astronomers could observe has grown over the last few centuries. Before 1609, astronomy was done solely with the human eye, which can resolve details as small as 1/50th of a degree. In 1609, Galileo pointed the first telescope at the heavens and was able to see about 600 times more than has ever been seen before. The world is celebrating the 400th anniversary of Galileo’s advance by declaring 2009 to be the International Year of Astronomy. As the chart shows, in the 400 years since Galileo, steady progress was made with ever more powerful telescopes. But all these pale in comparison to the dramatic advance achieved by the HST, which has provided almost 3 million times the information that can be seen by eye.

HST has opened a Golden Age of astronomy and opened our eyes to a more profound understanding of our universe and our place in it.

More on the telescope and the science later.

Now, we hope for the success and safe return of Space Shuttle crew Andrew Feustel, Michael Good, John Grunsfeld, Greg Johnson, Michael Massimino, Megan McArthur and Commander Scott Altman. These seven brave astronauts have trained for years and have accepted considerable risk to restore one of the greatest scientific instruments in human history.

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Monday, May 4, 2009

More about Cold Fusion

My earlier blog on cold fusion stirred up a hornets' nest of anonymous bloggers. Contrary to some of their allegations, I have no financial interest in “hot” fusion, nor do I work for an oil company. Indeed, I am retired and, unlike the cold fusion proponents on 60-Minutes, I have no financial interest in any specific company or technology in this field. Nor am I “merely” an academic. While I have physics degrees from Caltech and Stanford and was on the faculty of Harvard, I spent most of my career developing practical applications of advanced technologies, such as medical equipment that has dramatically improved patient care. I am sincere in wanting the best for our society, which is why I blog using my real name rather than hiding behind an alias.

I would be thrilled if someone developed a real “silver bullet” that instantly solved our critical energy and environmental challenges. I would also be thrilled if someone cured cancer, but I’m not rushing to buy snake oil from those eager to cash in on an unsuspecting public desperate for an easy cure.

If cold deuterium fusion were real it would be easy to provide definitive scientific proof, and that technology would be rapidly adopted. What will stop these people is not what I or anyone else says but rather they will fail because what they claim is simply not true.

I’ve read their statements and do not doubt that heat is released by their chemical reactions – there is nothing remarkable about that; matches do that also. Nowhere do they address the fundamental scientific issue that the energies required to fuse nuclei are vastly greater than those in any chemical reaction. They provide no evidence that helium is produced by the deuterium fusion they claim to achieve. There is no confirmation by an independent group that doesn’t stand to make money on cold fusion. As an old TV commercial once said: “Where’s the beef?”

Very promising, real solutions to our energy and environmental challenges exist, including solar, wind, and true fusion. All of these need major investments and decades to develop and implement. Let’s not let false hope and self-promoters divert us from investing our resources to achieve real progress – conservation, pollution-reduction, and new scientifically sensible energy generation.