CLOSE

The Expanding Universe: How the Universe Got Bigger As We Measured It

Since before history began, we have tried to understand our world and our place in it. To the earliest hunter-gatherer tribes, this meant little more than knowing the tribe's territory. But as people began to settle and trade, knowing the wider world became more important, and people became interested in the actual size of it. Aristarchus of Samos (310-230 BC) made the earliest surviving measurements of the distance between objects in space. By carefully measuring the apparent size of the Sun and Moon and carefully observing the terminator of the Moon when half full, he concluded that the Sun was 18-20 times farther away than the Moon. The actual value is 400, but he was on the right track; he just didn't have precise enough measurements.


A diagram from Aristarchus' work, "On Size and Distances," describing how to work out the relative distances.

Meanwhile, Eratosthenes of Cyrene (276-195 BC) was working on the size of the Earth. He came upon a letter stating that at noon in Syene (modern-day Aswan) on the summer solstice, one could look down a well and see all the way to the bottom because the Sun was precisely overhead. Eratosthenes already knew the distance between Alexandria and Syene, so all he had to do was observe the angle of the Sun on the summer solstice there and then do a little math. Assuming a spherical Earth, he computed the circumference to be 252,000 stadia, which works out to 39,690 km -- which is less than a 2% error compared to the real value. A directly measured size now existed for the world. But what of the heavens? The work of Aristarchus wasn't accurate enough. After figuring out how to reliably predict eclipses, Hipparchus (190-120 BC) used them to get a better estimate of the ratio of distance between Moon and Sun. He concluded that the Moon was 60.5 Earth radii away, and the Sun was 2,550 Earth radii away. His lunar distance was pretty accurate -- that works out to 385,445 km to the Moon, which is pretty close to the actual distance, an average of 384,400 km -- but for the Sun it worked out to 16 million km, about 136 million km short of the actual distance.

Above left: A dioptra, a predecessor to both the astrolabe and the theodolite, of a type similar to the one Hipparchus used to make his measurements.

When Ptolemy (AD 90-168) came along, the Universe shrank for a while.

Using the epicycles he assumed must exist within his geocentric universe, he estimated the distance to the Sun to be 1,210 Earth radii, and the distance to the fixed stars to be 20,000 Earth radii away; using modern values for the Earth's average radius, that gives us 7,708,910 km to the Sun and 127,420,000 km to the fixed stars. Both of those are woefully small (Ptolemy's universe would fit within the orbit of Earth), but they get even smaller if we use his smaller estimate for the Earth's circumference -- he estimated the Earth to be about 1/6 the size it actually is. (And therein hangs a tale, for Christopher Columbus would try to use Ptolemy's figure when plotting his journey west to the Orient, rather than the more accurate ones that had been developed in Persia since then.)


Ptolemy's world; at the time, the best map that existed of the known world.

By the end of the 16th Century, the size of the Earth was pretty well defined, but the size of the Universe remained challenging. Johannes Kepler solved the puzzle of orbital motion and calculated the ratio of the distance between Sun and various planets, enabling accurate predictions of transits. In 1639, Jeremiah Horrocks made the first known observation of a transit of Venus. He estimated the distance between Earth and the Sun at 95.6 million km, the most accurate estimate to date (and about 2/3 the actual distance). In 1676, Edmund Halley attempted to measure solar parallax during a transit of Mercury, but was unsatisfied with the only other observation made. He proposed that further observations be made during the next transit of Venus, in 1761. Unfortunately, he did not live that long.



Jeremiah Horrocks, observing the transit of Venus by the telescopic projection method.

In 1761, acting on the recommendations of the late Edmund Halley, scientific expeditions set out to observe the Transit of Venus from as many places as possible. More expeditions set out in 1769 for the second transit of the pair, including a famous journey by Captain James Cook to Tahiti, and in 1771, Jerome Lalande used the data to calculate the Sun's average distance as 153 million km, far larger than previously estimated, and the first time the measurement was close to right. Further transits in 1874 and 1882 refined the distance to 149.59 million km. In the 20th Century, it has been refined further using radio telemetry and radar observations of the inner planets, but it has not strayed much from that value. The size of the solar system was now known.

Above left: Sketch depicting the transit circumstances, as reported by James Ferguson, a Scottish self-taught scientist and inventor who participated in the transit observations.

But the universe is bigger than the solar system. In the 1780s, William Herschel mapped the visible stars in an effort to find binary stars. He found quite a few, but he also worked out that the solar system was actually moving through space, and that the Milky Way was disk shaped. The galaxy, which was at that time synonymous with Universe, was eventually estimated to be about 30,000 light years across -- an inconceivably large distance, but still far too small.

Hershel's map of the galaxy could not tell how far away any of the stars were; stars get dimmer as they move away, but you can only use this to calculate their distance if you know how bright they are to begin with, and how can you know that? In 1908, Henrietta Leavitt found the answer: she noticed that Cepheid variable stars had a direct relationship between their luminosity and the period of their variation, allowing astronomers to deduce exactly how bright they are to start with. Harlow Shapley immediately applied this discovery and found three amazing things when he mapped all the visible Cepheids: the Sun is actually nowhere near the center of the galaxy, the center of the galaxy is obscured by vast amounts of dust, and the galaxy is at least ten times larger than anyone had ever suspected -- so vast that it would take light 300,000 years to cross it. (Shapley was overestimating a bit; it's actually more like 100,000 light years or so.)

Above left: Henrietta Leavitt, one of the few women in astronomy and the only one on this list; she got little recognition for her discovery at the time.

In 1924, Edwin Hubble produced the next major revolution. Using the new 100-inch telescope at Mount Wilson Observatory, he located Cepheids in the Andromeda Nebula, a spiral nebula in which no stars had previously been resolved. He calculated these Cepheids were 1.2 million light years away, putting them far beyond Shapley's wildest estimate for the size of the galaxy. Therefore, Andromeda was not a part of our galaxy at all; it was an entirely separate "island universe," and most likely the same was true of other spiral nebulae. This meant the Universe was very likely far larger than anyone could hope to measure. It could even be infinite.

At left: The 100-inch telescope at Mount Wilson Observatory, where Hubble did his work. It was the world's largest telescope until 1948.

And then Hubble found something even more astonishing. In 1929, Hubble compared the spectra of near and far galaxies, based on distances already known by observations of Cepheid variables. The spectra of more distant ones were consistently redder, and for nearly all of them, there was a linear relationship between redshift and distance. Due to the Doppler Effect, this meant they were receding. He wasn't sure what to make of this observation at the time, but in 1930, Georges Lemaître pointed out a possible solution: he suggested that the universe was expanding, carrying galaxies along with it, and that at one time it had all be compacted down impossibly tight. Hubble went with this and calibrated the apparent expansion against the distance to known standard candles, calculating the age of the most distant objects to be 1.8 billion light years.

At left: Georges Lemaître, who happened to also be a Catholic priest. He died in 1966, shortly after learning about the Cosmic Microwave Background radiation, which further reinforced his theory of the Big Bang.

This was much too small, and in 1952, Walter Baade figured out why: there are actually two kinds of Cepheids, and Hubble had been observing the ones that Leavitt had not baselined. After characterizing this new population of Cepheids, he recalculated from Hubble's observations and brought the Universe's minimum age up to 3.6 billion years. In 1958, Allan Sandage improved it more, to an estimated 5.5 billion years.

Astronomers started to ratchet up their observations of ever more distant objects. In 1998, studies of very distant Type 1A supernovae revealed a new surprise: not only is the universe expanding, but the rate of the expansion is increasing. Today, the Universe is usually estimated to be 13.7 billion years old -- or, more accurately, the most distant things we can observe appear to be that far away. The catch, of course, is that we're observing them in the past. They're actually further away now -- assuming, of course, that they even still exist. A lot can happen in 13.75 billion years. And now that we know the universe's expansion is accelerating, they are even farther away by now. The current estimate for the actual size of the observable universe is 93 billion light-years in diameter, a tremendous size that the human brain cannot begin to fathom on its own, vastly overwhelming the tiny universe of the ancient Greeks.


NASA artist's concept of the progenitor of a Type 1a supernova -- a neutron star stealing matter from a supergiant companion until eventually enough matter is collected to trigger a supernova.

The understanding of the size of the Universe has gone from being impressed by the distance to the Sun, to the size of the solar system, to the vastness of the galaxy, to the staggering distance to neighboring galaxies, to the mindbendingly complicated distances to things that we can only see as they were an impossibly long period of time ago. What will we discover as we measure the Universe tomorrow?

nextArticle.image_alt|e
iStock
arrow
Space
Mysterious 'Hypatia Stone' Is Like Nothing Else in Our Solar System
iStock
iStock

In 1996, Egyptian geologist Aly Barakat discovered a tiny, one-ounce stone in the eastern Sahara. Ever since, scientists have been trying to figure out where exactly the mysterious pebble originated. As Popular Mechanics reports, it probably wasn't anywhere near Earth. A new study in Geochimica et Cosmochimica Acta finds that the micro-compounds in the rock don't match anything we've ever found in our solar system.

Scientists have known for several years that the fragment, known as the Hypatia stone, was extraterrestrial in origin. But this new study finds that it's even weirder than we thought. Led by University of Johannesburg geologists, the research team performed mineral analyses on the microdiamond-studded rock that showed that it is made of matter that predates the existence of our Sun or any of the planets in the solar system. And, its chemical composition doesn't resemble anything we've found on Earth or in comets or meteorites we have studied.

Lead researcher Jan Kramers told Popular Mechanics that the rock was likely created in the early solar nebula, a giant cloud of homogenous interstellar dust from which the Sun and its planets formed. While some of the basic materials in the pebble are found on Earth—carbon, aluminum, iron, silicon—they exist in wildly different ratios than materials we've seen before. Researchers believe the rock's microscopic diamonds were created by the shock of the impact with Earth's atmosphere or crust.

"When Hypatia was first found to be extraterrestrial, it was a sensation, but these latest results are opening up even bigger questions about its origins," as study co-author Marco Andreoli said in a press release.

The study suggests the early solar nebula may not have been as homogenous as we thought. "If Hypatia itself is not presolar, [some of its chemical] features indicate that the solar nebula wasn't the same kind of dust everywhere—which starts tugging at the generally accepted view of the formation of our solar system," Kramer said.

The researchers plan to further probe the rock's origins, hopefully solving some of the puzzles this study has presented.

[h/t Popular Mechanics]

nextArticle.image_alt|e
NASA
arrow
science
The Ozone Layer Is Healing, Thanks to an International Ban on Harmful Man-Made Chemicals
NASA
NASA

The ozone layer is on the mend, thanks to a decrease in human-produced chemicals called chlorofluorocarbons, or CFCs, in the atmosphere. Using data from NASA's Aura satellite, scientists were able to measure the chemical composition of the thinned gas layer above the Antarctic and found about 20 percent less ozone depletion than there was in 2005. They published their findings on January 4 in the journal Geophysical Research Letters.

In 1985, UK scientists published a landmark study in the journal Nature announcing their discovery of an annually recurring hole in the ozone layer above Antarctica. (Each September, as the Southern Hemisphere's winter arrives, the Sun's UV rays trigger a reaction between the ozone and chemical elements from CFCs, chlorine and bromine, which destroys the ozone molecules.) The finding led to the Montreal Protocol in 1987, an international treaty that gradually banned the production and use of CFCs in refrigerants, aerosol sprays, solvents, and air conditioners.

In July 2016, Antarctic researchers published a study in the journal Science reporting that the ozone layer appeared to be healing (although it wasn't projected to completely patch up for decades). They tracked this progress by monitoring the Antarctic ozone hole's area, height, and chemical profile. Still, they didn't know whether this progress could be attributed to the Montreal Protocol's mandate.

NASA itself has used Aura to monitor the hole since the mid-2000s. After analyzing data produced by the Microwave Limb Sounder, a satellite instrument aboard Aura that measures trace gases, the space agency has confirmed that the CFC ban has led to the big decrease in ozone depletion during the Antarctic winter.

By winter, ozone-busting chlorine compounds have converted into hydrochloric acid, a process that occurs after it's destroyed ozone particles and reacts with methane. "By around mid-October, all the chlorine compounds are conveniently converted into one gas, so by measuring hydrochloric acid, we have a good measurement of the total chlorine," researcher Susan Strahan said in a NASA statement. Scientists compared these hydrochloric acid levels with nitrous oxide, which is similar in nature to CFCs but isn't diminishing in the atmosphere.

Their study is billed as "the first to use measurements of the chemical composition inside the ozone hole to confirm that not only is ozone depletion decreasing, but that the decrease is caused by the decline in CFCs," according to NASA. But while these initial results are promising, scientists say that the ozone layer's full recovery is still a long way off.

"As far as the ozone hole being gone, we're looking at 2060 or 2080,” study co-author Anne Douglass said. “And even then there might still be a small hole."

SECTIONS

arrow
LIVE SMARTER
More from mental floss studios