Gravity 9: The Evolving Universe

by Shane L. Larson

Of all the fundamental forces in Nature, gravity is the weakest. What do we mean by that? Let’s forego our usual thought experiments, and do something real to demonstrate this idea.

First, go eat a piece of pizza (or any other food you enjoy). This is the process by which you accumulate the eenrgy needed to make your body go. Without pizza, you wouldn’t be able to do anything.  Second, go stand in the middle of the room (where you won’t hurt yourself) and jump straight up in the air, as high as you can.

Energy from pizza, gives me the power to defy the gravitational pull of the entire planet.

What happened? Since I’m pretty sure most of you reading this aren’t superheroes and can’t fly, you probably ascended up in the air a bit, and then came back down to the floor. It’s an everyday sort of thing, completely ordinary. But this is science, and there are remarkable and deep truths hiding in the simplest of circumstances. So consider this:

Using some simple chemical energy, which your body gleaned by breaking down some food you ate, you were able to (momentarily) overcome the gravitational pull of the ENTIRE EARTH.

This is what we mean when we say gravity is weak. But despite this fact, it is fundamentally the most important force of Nature if we want to think about the Cosmos as a whole. It has no competitor on the largest scales imaginable, meaning that even with its weak ability, gravity is able to change the Cosmos over the long, inexorable flow of time. It made sense that general relativity could and should be used to consider the past, present and future of the Universe itself.

The night sky over the Pando Forest in central Utah. Pando is an 80,000 year old aspen grove — it has seen almost 30 million nights like this one, but very little has changed. The constellations change over thousands of years, but the sky is still full of stars, and the Milky Way still arches over the sky, giving the impression that the Universe is unchanging. [Image: Shane L. Larson]

In 1915, when Einstein first presented general relativity to the Prussian Academy of Sciences, there was precious little we knew about the Universe, though perhaps we didn’t realize it at the time. The only objects outside the solar system that we knew a lot about were stars, and many scientists (including Einstein) supposed the Universe was comprised entirely of stars. Einstein himself made one of the first attempts to use general relativity to describe the Universe. He considered the case where the Universe was uniformly filled by stars, and found a result that disturbed him — no matter what he tried, general relativity predicted the Universe would collapse. To counteract this, Einstein modified general relativity through the introduction of a “Cosmological Constant” that made the Universe slightly repulsive. The result was precisely what Einstein hoped to find, what he and most scientists thought the Universe was: static, unchanging in time. But great changes were afoot, being driven by our ability to see the Universe better than ever before.

Henrietta Swan Leavitt, in her office at the Harvard College Observatory. She made one of the most important discoveries in the history of astronomy: how to measure distances to a common type of star, known as a Cepheid variable.

In 1915, the largest telescope of the day was the 60-inch reflector on Mount Wilson, though it would be eclipsed two years later by the 100-inch Hooker Telescope, also on Mount Wilson. Enormous telescopes such as these were enabling us to probe the size of the Cosmos for the first time. The key to making those measurements was discovered by a pioneering astronomer at the Harvard College Observatory, Henrietta Swan Leavitt.

Delta Cephei is a naked eye star, in the lower corner of the constellation of Cepheus.

Leavitt was studying a class of stars known as Cepheid variables. Named for the archetype, delta Cephei, these stars are “radial pulsators” — they grow and shrink over time in a regular pattern over the course of many days. The observational consequence, if you are watching, is the brightness and the temperature the star changes. What Leavitt discovered was a regular pattern between the time it took a Cepheid star to change its appearance (its “period”), and its true brightness (its “luminosity”).

The brightness of Cepheid variables goes up and down over the course of many days. The curve here is for Delta Cephei, which changes brightness every 5.4 days. (TOP) The observed change in brightness is a direct result of the stars size pulsating in time.

How does that help you measure distances? Let’s imagine a simple example here on Earth. Suppose you have a 100 Watt lightbulb and a 10 Watt lightbulb side by side.  The 100 Watt bulb looks brighter — way brighter. This is the intrinsic brightness of the bulb — it is clearly putting out more energy than its smaller, 10 Watt companion, which you can easily discern because they are right next to each other.  This intrinsic brightness at a known fixed distance is what astronomers call absolute luminosity or absolute magnitude.

Is there any way to make the 100 Watt bulb look dimmer? Yes! You can move it farther away — the farther you move it, the dimmer it appears. In fact, you could move it so far away that the 10 Watt bulb you leave behind looks brighter! By a similar token, you can make it look even brighter by moving it closer! How bright something looks when you look at it is what astronomers call apparent luminosity or apparent magnitude.

Disentangling distance and brightness is one of the most difficult problems in astronomy. Observing how bright an object is depends on two things: its intrinsic brightness, and its distance. (TOP) A bright light and a dim light are side by side at a fixed distance; one is obviously brighter than the other. (BOTTOM) If the brighter light is farther away, it can look dimmer than the closer light!

By comparing apparent brightness (how bright something looks in a telescope) to absolute brightness (how bright something would look from a fixed distance away) you can measure distance. The biggest problem in astronomy is we don’t know what the absolute brightness of objects are.

What Leavitt discovered was if you measure how long it takes a Cepheid to change its brightness, then you know its absolute brightness. Comparing that to what you see in the telescope then let’s you calculate the distance to the star! This discovery was a watershed, arguably the most important discovery in modern astronomy: Leavitt showed us how to use telescopes and clocks to lay a ruler down on the Universe. Leavitt died of cancer at the age of 53, in 1921.

Harlow Shapley.

Despite her untimely death, astronomers rapidly understood the power of her discovery, and began to use it to probe the size of the Cosmos. Already by 1920 Harlow Shapley had used the Mount Wilson 60-inch telescope to measure Cepheids in the globular clusters in the Milky Way. What he discovered was that the globular clusters were not centered on the Earth, as had long been assumed, but rather at some point more than 20,000 lightyears away. Shapley argued quite reasonably that the globular clusters are probably orbiting the center of the galaxy. This was the first indication that the Copernican principle extended far beyond the Solar System.

In 1924, Edwin Hubble, who Shapley had hired at Mount Wilson Observatory, made a stunning announcement — he had measured Cepheid variables in the Andromeda Nebula, and it was far away. At 2.5 million lightyears away, the Andromeda Nebula was the farthest object astronomers had ever measured the distance to. In fact, it wasn’t a nebula at all — it was a galaxy. Here, for the first time, some of the long held, cherished beliefs about Cosmology that were prevalent when Einstein introduced general relativity began to unravel. (Historical Note: Hubble’s original distance to the Andromeda Galaxy was only 1.5 million lightyears. Why? Because there are two different kinds of Cepheids, both of which can be used to measure distances, but calibrated differently! Astronomers didn’t know that at the time, so Hubble was mixing and matching unknowingly. Eventually we learned more about the Cosmos and arrived at the current known distance — science is always on the move.)

The Hubble Ultra Deep Field (UDF), showing what can be found if you stare at an “empty” part of the sky for long enough. Virtually every object in this image is a distant galaxy.

The Universe was not full of stars…. it was full of galaxies, and those galaxies were further away than we had ever imagined. This was a dramatic discovery that shook astronomers deeply. But it was only the beginning. A scant five years later, Milton Humason and Hubble, using the 100-inch telescope at Mount Wilson, made another astonishing discovery: every galaxy they looked at was receeding away from the Milky Way, in every direction. Furthermore, the farther away the galaxy was, the faster it was receding from us.  This result is now known as “Hubble’s Law.” Humason and Hubble had stumbled on one of the great secrets Nature — the Universe was not static, as a casual comparison of the night sky from one year to the next may suggest.  But what was going on? Why were all the galaxies flying away from us, in every direction we looked? This would seem to contradict the Copernican principle that we weren’t the center of everything!

(L) Alexander Friedmann. (R) Georges Lemaître.

As it turns out, the answer was already in hand. It had been discovered several years before Humason and Hubble by two scientists who had sought to use general relativity to describe the Cosmos: Alexander Friedmann, a Russian physicist, and Georges Lemaître, a Belgian priest. Friedmann had used general relativity to describe a Universe that was homogeneous (the same everywhere) and isotropic (looks the same in every direction). The “Friedmann Equations,” as they are now known, describe the evolution of such a Universe as a function of time. Lemaître derived the same result in 1927, two years after Friedmann’s death. In the mid 1930’s, American physicist H. P. Robertson and UK physicist A. G. Walker showed that the only solution in general relativity describing a homogeneous and isotropic Universe as that of Friedmann and Lemaître. This is now called the FLRW (“Friedmann-Lemaître-Robertson-Walker”) Cosmology.

What the FLRW cosmology tells us is that the galaxies aren’t really flying apart from one another — if the Universe is homogeneous and isotropic, then spacetime itself is changing, stretching and deforming. The reason the galaxies are receding from one another is the spacetime between them is expanding — the Universe is getting larger, expanding all the time.

Imagine the galaxies floating in spacetime, unmoving with respect to one anther (they are stapled down to their location in spacetime). General relativity predicts that the galaxies don’t move, but that spacetime itself expands, it stretches, so the distance you measure between galaxies increases.

Lemaître was the first person to think differently about this problem. He had the presence of mind to ask, “We see the Universe is expanding, but what if I run time backward? What did the Universe look like in the past?” In 1931, he argued that the expansion seen in every direction suggested that the Universe had expanded from some initial point, which he called the “primeval atom.” If today we see everything expanding away, and you look backward in time, it must have all been much more compressed and compact, a state which would have made it hot, and dense. Lemaître didn’t know what might have initially caused the expansion of this primeval atom into the Cosmos we see today, but he did not see that as a reason to suppose the idea was invalid.

Lemaître with Einstein in California, 1933.

Change in science is hard, especially when data is new and our ideas are undergoing a dramatic evolution from past modicums of thought. Einstein is widely known to have critically panned both Friedmann’s and Lemaître’s work before the discovery of the expansion, still believing in the notion of a static Universe. Once the scientific community had come to understand and accept the expansion data, it required another great leap of faith to contemplate Lemaître’s notion of a hot dense initial state. Einstein again was skeptical, as was Arthur Stanley Eddington. For more than a decade, the arguments about the idea raged, and in 1949 during a BBC radio broadcast, astronomer Fred Hoyle coined the term by which Lemaître’s “primeval atom” idea would forever be known as: the Big Bang.

All ideas in science stand on equal ground — they are valid for consideration until they are proven wrong by observations. If the Universe did indeed begin in a Big Bang, then the obvious question to ask is what signatures of that dramatic event would be observable today? As it turns out, there are many observational consequences of the Big Bang, and they all have been observed and measured by astronomers, lending confidence to Lemaître’s initial insight.  This will be the topic of our next chat.

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This post is part of an ongoing series written for the General Relativity Centennial, celebrating 100 years of gravity (1915-2015).  You can find the first post in the series, with links to the successive posts in this series here: http://wp.me/p19G0g-ru.

Cosmos 7: The Backbone of Night

by Shane L. Larson

Science is a powerful method to know the world. Without science, the Cosmos would be an impenetrable mystery to us; we would live our lives without the benefit of knowing how to make fire, how to cross the water or fly in the sky, or how to combat fevers and illness. Early in our education, we are often instructed in The Scientific Method.  I remember long hours as a student, filling up lab notebooks with precisely organized and formatted records of investigations into the mysteries of Nature, followed by my professors mercilessly docking me points for not completely writing out an equipment list or forgetting to record a step of the procedure in my notebook — points lost for not following explicitly the formal steps listed in The Scientific Method. I’m sure those days of bleeding red ink on my lab notebook taught me to be more careful in my scientific records, but they did little to inspire me about the way to do science.

One of my lab notebooks, from my undergraduate days.

Now, many years into my career as a professional scientist, I still think I practice The Scientific Method, but in reality I find the process of science is more organic, moving forward on leaps of inspiration, mixed together with careful planning, fraught with confusion and difficulties, and punctuated with debate and argument. Science is a very human endeavour, and as such has all the elements of every story ever told: ignorance, curiosity, hubris, depression, discovery, elation, and ultimately wisdom.  One such story, spanning more than 400 years, is about how we have come to understand our own galaxy.

The Milky Way rising over Mt. Hood and Lost Lake, Oregon.
[Image by Ben Canales, http://www.thestartrail.com]

From remote dark skies, far from the bustling life of modern cities, you can see the Milky Way, arching overhead. Nebulous and shimmering, it has evoked wonder and questions for many thousands of human generations. While today the nature of the Milky Way has passed from our scientists into the collective knowledge base of our species, there was a time when the Milky Way was a tremendous mystery upon which we hung stories, myths and legends. The peoples of the Indian sub-continent called the Milky Way Akash Ganga, the Ganges of the heavens. Life along the Ganges is intimately tied to the river, and it is sacred to the Hindu people; perhaps staring at the vast gossamer river in the sky touched some deep chord in their minds, igniting the idea that we here on the Earth are connected to the sky.  The !Kung bushmen tribes of the Kalihari desert call the Milky Way The Backbone of Night, because it looks like a ghostly arch, soaring overhead and holding up the sky.

Galileo Galilei.

Our knowledge of the Cosmos is like an archway — it is built up stone by stone, and supported by keystones, essential pieces of knowledge that define how we think about Nature. The Milky Way has come to be a keystone, a focal point of attention that has guided us in our long journey to understanding the Cosmos. In our understanding of the galaxy, that journey began 400 years ago with a 45 year old professor from Padua, Italy, who turned a spyglass to the heavens — Galileo Galilei.  His telescope was a simple device, of poor imaging quality compared to the cheapest pair of binoculars you might find at a discount store today. But it could show more than just the eye alone.  Peering through his telescope at the diaphanous mist of the Milky Way, Galileo was presented with a staggering wonder — the galaxy was comprised of innumerable stars, packed so closely together that the eye could not resolve them, instead seeing only a nebulous fog. The Universe had suddenly gotten much larger.

There is a bit of folklore that in those early days, Galileo doubted what the telescope was showing him. How could he be certain that what he saw when he looked to the skies was real, and not some phantasm born of his mind’s inability to interact with his new-fangled optical device? To answer this question, he dutifully did what Galileo is known so well for — he conducted an experiment. Setting up a coin across a courtyard, he viewed and sketched the coin through his telescope, noting every detail he could see.  Then, leaving his telescope behind, he walked right up to the coin and sketched it again viewing it from a distance of only a few inches. After much examination, he convinced himself the telescope was not lying to him, and all the wonders he had seen were real.

A Venician silver scudo, from around the same era when Galileo was learning to use his telescope.

He published his first telescopic observations of the heavens in 1610, in a book called Sidereus Nuncius — “The Starry Messenger.”  Despite having convinced himself of the truth, he had less luck convincing others of the utility of the telescope.  He complained in a letter to Kepler, 

“…I think, my Kepler, we will laugh at the extraordinary stupidity of the multitude.  What do you say to the leading philosophers of the faculty here, to whom I have offered a thousand times of my own accord to show my studies, but who with the lazy obstinacy of a serpent who has eaten his fill have never consented to look at planets, nor moon, nor telescope?  Verily, just as serpents close their ears, so do these men close their eyes to the light of truth.  These are great matters; yet they do not occasion any surprise.”

Even in Galileo’s day, communicating science was hard.  But the Age of Enlightenment was soon upon the world; modern astronomy was born in this time, a direct descendant of Galileo’s stargazing.  Telescopes began to proliferate. Larger telescopes were built, new designs were invented, observatories were constructed, and astronomers were appointed.  We began to plumb the heavens, trying to see all that we could see.

As we looked deeper into the sky, we came to understand that there was far more to the Milky Way than even Galileo knew. The first person to map the Milky Way was William Herschel.  In 1784, Herschel used his telescope to count stars in every direction on the sky.  From those studies, he produced a map, eerily accurate to what we know today, concluding that the Earth and the Sun are near the center of a flat disk of stars (you can read a detailed description of Herschel’s method in this paper).  But as telescopes scanned back and forth across the sky, observers would occasionally see things that were not stars — new, dim fuzzy objects.  Some were round, some were oblong, some were completely irregular. The looked for all the world like the Milky Way looked before Galileo’s telescope — thin, white, diaphanous fogs among the stars. They were generically named nebulae, Latin for “clouds.”  Herschel himself catalogued more than 2400 of these in an epic survey of the sky conducted with telescopes he built.

(L) William Herschel, (R) Herschel’s first map of the Milky Way.

This first reconnaissance of the sky brought to the forefront of our minds questions we had asked before: how big is the Cosmos? where did it come from? what is our purpose in it? At this time, most astronomers had decided that the entire Universe was the Milky Way. They had no reason to believe (nor ability to measure) that distances in excess of thousands of lightyears were reasonable.  Thus all the nebulae, since they were parts of the Universe, must reside within the Milky Way itself.  One prominent view of the day was the “nebular hypothesis,” which supposed that stars and planets formed from gravity acting to collapse vast clouds of gas. The detection of nebulae among the stars of the Milky Way could explain where all the stars in the galaxy came from.

Today we now accept that the nebular hypothesis is correct, but in the 19th Century there were those who certainly did not. Among those who disliked the notion was William Parsons, the Third Earl of Rosse. He believed that like the Milky Way, the nebulae should resolve themselves into innumerable faint stars, if you could just look with a powerful telescope.  So in 1845 he built one of the largest telescopes the world had ever seen.  More than six feet in diameter, the structure of the telescope had to be held up by a castle wall, and was colloquially known as “The Leviathan of Parsonstown.”

(L) Lord Rosse, (R) The Leviathan of Parsontown (next to its support wall at Birr Castle).

One of the great truths of telescopes is that bigger telescopes can see more, because they collect more light.  By all accounts, the Leviathan could see more than any telescope of the time — it revealed details in the nebulae that had never been seen before.  In 1845, shortly after it was built, Lord Rosse pointed the Leviathan toward a distant nebula, just under the handle of the Big Dipper, known as Messier 51 (“M51”). In a moment that must have been utterly breathtaking, Lord Rosse realized that he could see spiral structure in the nebular cloud. He promptly declared that M51 was an “island universe,” another galaxy like the Milky Way.

(L) Lord Rosse’s first sketch of M51 in 1835, showing the spiral structure seen through the Leviathan. (R) A modern HST image of M51 [NASA/ESA].

This ignited “The Great Debate” in astronomy, which persisted for more than 80 years before it was resolved.  Arrayed against each other were the scientists who believed the Milky Way was the entire Universe, versus those who contended that the Milky Way was but one of a vast number of other galaxies.  Both sides had good arguments that supported their position, but there was no way to decide that one group was more right than the other — the observations simply weren’t good enough.  To resolve this debate we had to know two things: the size of the Milky Way itself, and the distance to the spiral nebulae.  Astronomers noodled this over in vain for decades to no avail.

Henrietta Swan Leavitt at her desk [Harvard College Observatory].

In the end, the solution to the problem was discovered in 1912 at the Harvard College Observatory by astronomer Henrietta Swan Leavitt.  She had discovered a type of star now known as a Cepheid variable that changes its brightness in a regular way over time (a variable star). Leavitt demonstrated that for Cepheid variables, if you could accurately measure the time it takes the star to get dim then bright again, you could use the change in brightness to determine the distance to the star.  This was the first, robust method for using telescopic observations of stellar brightness to determine distances through the galaxy; Leavitt’s discovery transformed astronomy.  Sadly, Leavitt died of cancer at the age of 53 in 1921, before The Great Debate was resolved.

Hubble at the eyepiece of the 100″ Hooker Telescope on Mount Wilson [Time & Life Pictures/Getty Images].

Knowing that bigger telescopes see more, the Mount Wilson Observatory built a 100” telescope overlooking Los Angeles in 1917; it would be the largest telescope in the world for more than 30 years.  In 1924, Edwin Hubble announced that he had used the 100” telescope to detect Cepheid variables in several spiral nebulae. Using Leavitt’s discovery, he was able to determine the distance to the spiral nebulae, discovering that they were vastly farther away than astronomers had imagined.  The Universe was suddenly a very big place!

How big? The Milky Way galaxy is about 100,000 lightyears across — it takes light 100,000 years to travel from one side to the other. The disk, which we see edge on as the faint river of light in the night sky, is on average only about 10,000 lightyears thick.  By contrast, Hubble was observing the Andromeda Nebula, which is 2.5 million lightyears away!  It would take 25 Milky Way galaxies laid edge to edge to span the gulf of space to our closest neighbor, and there are  galaxies further still.  While these vast distances startled astronomers, The Great Debate was, for all practical purposes, resolved instantly. The data was clear and unambiguous. Leavitt’s great breakthrough in the discovery of the Cepheid variables was a singular event — it resolved an argument that had plagued and befuddled us for almost a century.  Astronomers shook hands, dusted off their chaps, and moved on to new, equally difficult mysteries, suddenly revealed by uncountable galaxies far, far away.

Our current best understanding of the structure of the Milky Way, if it could be seen from above the galaxy. [Image by European Southern Observatory].

How do we study those galaxies that are so far, far away? We build bigger telescopes. We look at the Milky Way up close, and assume galaxies far away are similar. We spend time being confused. We argue. We make inspirational breakthroughs, and eventually, we understand.  This is the nature of science.

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This post is part of an ongoing series, celebrating the forthcoming science series, Cosmos: A Spacetime Odyssey by revisiting the themes of Carl Sagan’s classic series, Cosmos: A Personal Voyage.  The introductory post of the series, with links to all other posts may be found here:  http://wp.me/p19G0g-dE