Why does Hubble’s Law often confound those born and raised on Earth?
A wide variety of modern astronomical observations clearly show that our Universe had a beginning in time, known as the “big bang.” But, those same observations also show that the Universe expands in all directions simultaneously, meaning that it has no single birthplace in space. Unless time is treated as a fourth dimension, akin to the three dimensions of space, these ideas--a starting time, but not a starting place, are difficult for humans who grew up on Earth, where time and space seem separate concepts, to reconcile. An Earth-centric upbringing motivates two very “frequently-asked” questions about an expanding Universe that began with a big bang:
1. If the Universe had a beginning, what was here before the big bang?
2. If space is expanding, what’s it expanding into; what lies beyond?
Of course, these two tricky questions mentioned above can be avoided if a supernatural power pressed a "start" button on time, and controls whatever is "beyond" our Universe. In Ancient times thinkers across cultures believed the Universe was controlled at least in part by supernatural forces.
Even by the 17th century, when Galleo’s telescopic observations of Jupiter had effectively disproven the Ancients’ Earth-centered Solar System, and Isaac Newton had shown how gravity controlled the motions of the planets as they orbit the Sun, supernatural explanations were still popular. Even Newton was in large part searching for ways to understand God. You can learn more about this history in The Path to Newton, an interactive infographic describing the history of ideas leading to Newton’s theory of gravity.
|Hubble’s famous image of M31 (a.k.a. the Andromeda Galaxy or NGC 224) where he marked a star as “VAR!” for “variable” in red ink. More at this site: Hubble's Famous M31 VAR! plate). Image courtesy of Carnegie Observatories.|
No one wanted the Universe to change much.
While thinkers prior to Hubble’s era disagreed on whether the Universe was infinite (Anaximander) or finite (Olbers), they essentially all agreed that it was unchanging (Aristotle) at some great distance from Earth.
Even during the Renaissance, which saw a great expansion of scientific ideas, the appearance of Tycho’s supernova in 1572 was tremndously unsettling to natural philosophers because it meant that something about the Universe--even if just the brightness of a single star, could change.
Centuries later, even Einstein, whose equations all but demanded
change, famously hated the idea of anything but a static Universe.
What changed people's thinking?
In the 19th century, photography and spectroscopy entered astronomy,
and ultimately changed everything. Long exposure photographs revealed scores of beautiful nebulae and millions of previously unknown faint stars.
Spectroscopy showed clear evidence that many of the elements we see on Earth are also in space, and it let us, via Doppler shifts, measure how fast any object on the sky is moving toward or away from us.
How big is the Milky Way - is it the whole Universe?
A modern Hubble Space Telescope view of M31,
(which looks much more obviously “galaxy-like” to
astronomers). Source of image
If you’re curious to see where M31 is on the Sky,
click here to see the “VAR!” portion of Hubble’s original
image in content in WorldWide Telescope.
By the early 20th century, it was possible to see and measure a very different Universe than was accessible to the Ancients, or even to Galileo and Newton. It had many more stars, and it had beautiful collections of stars (then called “nebulae”) with crazy patterns—like spirals. (See the modern image of M31 shown here in color, for a good example of a “Spiral” galaxy.) Spectroscopy hinted that faraway objects were composed of elements not unlike those in the Sun, and that they were moving with large speeds. Thanks to the explosion of new photographs and spectra, debate and uncertainty about the true size and extent of the Universe began to spread. Famously, in 1920, Harlow Shapley and Heber Curtis debated whether observed “spiral nebulae” were just at the edge of our Milky Way, which essentially was the whole Universe (Shapley) or were in fact their own “island universes” in sea of galaxies, of which the Milky Way was just one (Curtis).
Portrait of Henrietta Leavitt
from AAVSO, via AP
Harlow Shapley took the incorrect Milky-Way-is-everything position in 1920. This position seems odd in hindsight, given that only a short while earlier (in 1912) Henrietta Leavitt, a colleague of Shapley’s at the Harvard College Observatory, published what is now known as “Leavitt’s Law,” that allows measurement of distances using a relation between periodic changes in the brightness of Cepheid variable stars and their intrinsic brightness. Shapley in fact over-estimated distances in his work, but still clung to a gigantic Milky Way being the full Universe. Somewhat ironically, it was also Leavitt’s Law that let Edwin Hubble, who shared Heber Curtis’ view that spiral nebulae were galaxies beyond the Milky Way, measure the distances that later unlocked the secrets revealed by the Hubble Law.
OK, so there are LOTS of other galaxies beyond the Milky Way.
Are they moving?
|Portrait of Edwin Hubble with his image of M31, from Brittanica|
Spectroscopy, which can identify objects via unique combinations of so-called “spectral lines,” can also be used to measure velocity, thanks to the Doppler effect. Leavitt used the “identification” properties of spectroscopy to find her law, but to find his, Hubble needed to focus on velocity measurements as well. He made extensive use of the measurements of Vesto Slipher, who was the first astronomer to measure Doppler shifts to higher velocity (known as “redshifts”) for distance galaxies, and to realize that meant the galaxies were moving away from us.
Thus the stage was set for the revolution that Hubble’s Law would effectively begin. Astronomers were still arguing about the true size of the Universe, and most, including Einstein, wanted it to be static. But, then came Hubble’s 1929 result, showing a “linear” trend for galaxy velocity to increase with the separation of that galaxy from our own. The simplest explanation for galaxies moving faster and faster the farther apart they are is an expanding Universe--one where space and time are connected, and space itself expands as time moves forward. No single galaxy, or place, is the “center” of the expansion, as Hubble’s Law would look the same as viewed from any galaxy.
The Raisin Cake Analogy
Raisin Cake Video Link: https://youtu.be/-xrg8Ikanf8
A traditional analogy used to describe the motions of galaxies in an expanding Universe is to a raisin in a baking cake. This is a great analogy in that each raisin moves, on average, away from all the other raisins as the cake bakes and expands. BUT, this is a terrible analogy when it comes to the two questions we are trying to address here:
1. If the Universe had a beginning, what was here before the big bang?
2. If space is expanding, what’s it expanding into? what lies beyond?
“Before” and “beyond” have meaning for a baking cake that exists in a 3-D pan in space, and that changes in time, where time as a separate dimension from space. But “before” and “beyond” do not have analogous meanings in 4D space-time that is constantly expanding in both space and time.
The Birth of Modern Cosmology
To really understand the Universal expansion that Hubble’s 1929 Law implied took very open minds–ones able to think in abstract mathematical terms about 4D space time. Eventually, nearly all 20th-century astronomers came to grips with a huge, and expanding Universe.
Today, the study of the origins and expansion of the Universe is known as “cosmology.” The challenges facing modern cosmologists are well-captured in small revisions to Hubble’s Law made possible by ever-more accurate and abundant data. In particular, we now know that the expansion of the Universe is actually accelerating, in such a way that Hubble’s straight-line fit to velocity vs. distance actually curves upward a bit. Einstein knew that such acceleration was possible, so he had added his famous “cosmological constant” to keep the Universe static. Today, we name the mysterious cause of the Universe’s acceleration “dark energy,” and there is no good theory--yet--to explain it or its origin.