When the universe expanded tremendously after the Big Bang, the resulting gravity waves interacted with the cosmic microwave background to produce this characteristic “B-mode” pattern. Credit: BICEP2 CollaborationBig news in the cosmos today! Researchers from the BICEP2 south pole telescope have found ancient proof that the universe expanded tremendously after the Big Bang, a theory known as inflation. The discovery tells us (albeit indirectly) about an even earlier stage of the universe than we’ve ever before observed, and it provides crucial evidence that inflation did indeed occur. In so doing, it extends our model of the early universe from about one second after the Big Bang right back to less than 10-37 seconds after the event — a stunning leap forward (or backward, as the case may be).
To understand this, let’s back up 13.8 billion years or so, to the Big Bang. Also known as the birth of the cosmos and the origins of time and space, this burst of everything set the universe in motion. But a few niggling issues cast some doubts on the Big Bang theory — one of which was the mystery of how the universe came to be so uniformly spread out.
Enter the idea of inflation, in 1980, which suggested that just a few instants after the big moment, the universe suddenly grew enormously. This addition to the cosmic timeline explained why the universe was relatively uniform and it fit nicely with what we already knew about the universe’s earliest moments. However, cosmologists had no direct proof of inflation.
One way to prove inflation occurred, physicists thought, would be to look for gravitational waves created in its wake. These are basically ripples in the “fabric” of space-time — what the universe is made out of. Gravity is a relatively weak force, though, so we could only hope to detect the largest waves out there, caused by huge interactions like black holes colliding. Even though inflation was a relatively huge thing — it literally shaped the whole universe — the gravity waves it produced are now too weak to measure directly.
So instead, researchers were looking for the effect of inflation’s gravity waves on light. And not just any light, but the cosmic microwave background, “echoes” of light leftover from the Big Bang’s energy, created when the universe was just 380,000 years old. When this light interacted with the gravity waves, the theories said, it would have produced a distinctive pattern, called the B mode, in the light’s polarization. Such a pattern would be direct evidence that the gravity waves caused by inflation were real, and thus a key proof of inflation. And today, scientists announced they’d found it.
Assuming the finding is confirmed (and that looks likely — the team apparently spent 3 years going over their own data to make sure it was sound before coming forward with it), that’s huge news for cosmology. Direct evidence for inflation has been sought after for decades. Nature quotes Alan Guth, the main “inventor” of inflation, as saying, “This is a totally new, independent piece of cosmological evidence that the inflationary picture fits together,” and adding that the findings are “definitely” Nobel prize-worthy.
But it’s also big news for a couple of other reasons. First, in addition to being the first evidence for inflation, it’s also the first direct evidence for gravitational waves. Even though some observatories have been (and will continue!) looking for these gravitational waves, they’re still incredibly hard to find. The more data we have on these weird, space-time warping ripples, the more we’ll be able to understand the universe itself, and this is a great step in that direction.
And the other bit of significance to this has to do with understanding gravity in the first place. It’s currently the only one of the four fundamental forces not to play nice with quantum mechanics, which explains how things work on the tiniest scales. At high temperatures (like those found shortly after the Big Bang), the other three even begin to unify into a single super-force. One of the biggest issues in physics today is figuring out how (or if) gravity fits into this picture, and the findings that gravitational waves can result from inflation, a fundamentally quantum phenomenon, suggests that quantum gravity might indeed be possible.
A glimpse into the very first milliseconds of our universe, plus bigger questions ahead — all in all, it’s a pretty good day for science.
Fig. 1 Thermochronology data from the Grand Canyon region. (A) Map of the Grand Canyon region showing apatite helium samples discussed in the text (1, 13–15). (B) Carving of an Eastern paleocanyon from 25 to 15 Ma is indicated by different temperatures of rim- and river-level samples until ~25 Ma. (C) Western Grand Canyon thermal models are in conflict, but joint inversion of AFT and AHe data [purple curves, from (14)], suggest that the western Grand Canyon was carved in the the past 6 million years. (D) The top left diffusion profile (1) may fit the “young canyon” model if modeled without the highest temperature step. (E) Full data set of AHe ages (top) resembles predicted “young” canyon distribution of (1). Joint inversion of independent AHe and AFT data sets is especially powerful and provides well-constrained cooling histories for river samples in the eastern Grand Canyon (14); these show that basement rocks cooled slowly from 80° to 70°C between 65 and 25 Ma, then cooled rapidly from 25 to 15 Ma. The geometry of their published rim-level samples (shown in our Fig. 1A) is not optimal for resolving paleocanyons, but all available data (12–15) suggest that rim- and river-level samples, now separated vertically by 1 to 1.5 km, resided at 45° to 55° and 80°C, respectively, from 60 to 25 Ma. There is no evidence for a paleocanyon until after 25 Ma, when rim- and river-level cooling paths converge (Fig. 1B). Similar data show that the Marble Canyon section of the eastern Grand Canyon was buried by ~2 km of rock, and hence no canyons existed there until after 10 Ma (14). The combined data (Fig. 1B) refute the hypothesis for carving of the eastern Grand Canyon by 55 Ma (1, 2). The western Grand Canyon cooled earlier than the eastern Grand Canyon because of its proximity to the ancient Sevier/Laramide highlands. This region was eroded by northeast-flowing Laramide paleocanyons (9) and is cut by numerous faults with a history of recurring movement (12). A model from one 4He/3He sample (CP06-69) (Fig. 1C) suggests that rocks cooled to <30°C (~200 m depth) and have resided at these cool temperatures since 70 Ma (1). However, this interpretation conflicts with the joint inversion of AFT and AHe data from nearby samples (14), which suggests that these rocks cooled from ~60° to 40°C between 60 and 25 Ma (01-GC86) (Fig. 1C), compatible with ~1-km burial depth (the present depth below the rim). These conflicting results (1, 14) have several plausible explanations: (i) Sample “ensembles” from (1) span several known faults and therefore may not have shared a common cooling history. (ii) Western Grand Canyon samples accumulated considerable radiation damage during residence in the AHe partial retention zone for >600 million years and may not have been heated enough during the Cretaceous time to fully anneal grains, such that western Grand Canyon models should be rerun starting ~600 Ma to account for any incomplete annealing and inherited helium. (iii) When the combined AFT and AHe data sets (1, 12–14) are merged, the results of (1) are more closely reproduced by the “young” canyon than the “old” canyon model (Fig. 1E). The conflicting models (Fig. 1C) could both be correct if (iv) sample CP06-69 (1) was situated beneath a north-flowing paleocanyon near Separation Canyon, whereas sample 01GC-86 (14) was from an interfluve; or (v) CP06-69 was cooled on the upthrown side of an unrecognized Laramide reverse fault relative to 01GC-86. Although our knowledge of the north-flowing Laramide paleocanyon system is incomplete, existing thermochronologic data argue against a 70-Ma western Grand Canyon that followed the same path with nearly the same depth as the modern canyon. A simple dichotomy of “old” canyon versus “young” canyon hypotheses is overly simplistic because the Grand Canyon includes different sections with different geologic histories. Older paleocanyons likely were reused or re-excavated once the river found its modern path and began eroding rapidly. Despite these complexities, existing data do not support the model for a 80- to 70-Ma northeast-flowing California river, nor a 55-Ma southwest-flowing Arizona river, that collectively carved the Grand Canyon to within a few hundred meters of its modern depth by Early Tertiary time. Instead, an overwhelming body of published geologic and thermochronologic evidence shows that a majority of the Grand Canyon—the canyon that we see from the rim today—has been carved in the past 5 to 6 million years by the Colorado River. Drainage integration at 5 to 6 Ma was likely facilitated by older paleocanyon segments, whose geometry is now coming into focus. Received for publication 12 December 2012. Accepted for publication 25 February 2013. ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? Acknowledgments: Funding for the University of New Mexico coauthors (K.E.K., R.C., L.C., and J.W.R.) was from NSF EAR-0711546 and EAR-1119629. 
