How Was the Universe Born?

None of us were around to witness the violent birth and explosive early growth of the universe some 13.8 billion years ago — and that’s a good thing. At first, the dense, fast-expanding universe was full of superheated electrons and protons whizzing around, creating a sort of murky, searingly hot soup. But roughly 400,000 years after the Big Bang, matter cooled enough for those particles to coalesce into atoms. For the first time, light streamed freely through the cosmos.

The light released at that moment is the oldest cosmic event that we can measure. Scientists discovered this “cosmic microwave background” in 1965 and have scoured it ever since for clues about how the universe was born. Though at first glance it resembles a dull monotone, the microwave background is in fact a collage of slightly hotter and colder regions, representing denser and sparser accumulations of primordial matter that may have seeded the first galaxies.

Scientists study the cosmic microwave background to answer a question as old as science itself: How did we get here? They hope to uncover clues about what happened in the universe’s first moments to create the varied, textured universe we inhabit today. According to the leading theory, the baby universe burst onto the scene in an event called the Big Bang, then inflated explosively over a fraction of a second. If this “inflation” theory is correct, tiny fluctuations present at the beginning of time could have been magnified into much larger structures. In the process, they would have unleashed a torrent of powerful gravitational waves that rocked the young universe and eventually left their imprint on the cosmic microwave background. 

Scientists examine the cosmic microwave background’s faint, diffuse light for traces of these primordial gravitational waves, which would have imparted a subtle but distinct twisting pattern in the light. If this pattern, known as B-mode polarization, is found, it would provide strong evidence for cosmic inflation and vindicate a decades-long scientific quest.

Such measurements could also provide a unique experimental probe of quantum gravity. For 100 years, physicists have tried to unify two leading theories: general relativity, which describes gravity and acts on large scales, and quantum mechanics, which describes the universe at its smallest and most fundamental scales. 

So far, the theories of the very large and the very small have refused to play nicely together. But because gravity is so weak, no experiment has been able to measure its effects at the quantum scale. During the Big Bang when all matter was compressed into a tiny volume, quantum mechanics would have acted directly on the gravitational field. Studies of the cosmic microwave background could therefore start to reveal what happens when quantum and gravity meet.

What Is NIST’s Role?

Light from the cosmic microwave background has been traveling and diffusing through the cosmos for 13.8 billion years. It represents some of the faintest signals in the sky. So to study it, astronomers must gather every bit of light they can. This quest has pushed the frontiers of measurement science — and NIST has been a leader from the beginning.

To tease out the microwave background’s wispy, ghostlike signals, NIST scientists build specialized sensors that can capture and measure individual particles of light known as photons. These sensors are made of thin films of material with a special property: When chilled to frigid temperatures below around one-tenth of a degree above absolute zero, the material becomes a superconductor — it conducts electricity with no resistance. The temperature below which a material becomes superconducting is called its critical temperature. 

To operate these sensors, researchers apply a small voltage while keeping the chilled detector poised between two physical states: one in which it acts as a superconductor and one in which it is a normal electrical conductor with resistance. When a sensor absorbs energy from incoming light, the film material heats up and tips into the nonsuperconducting state. The films’ electrical resistance jumps, reducing the current flowing through it. This plunge in current indicates that a photon has been detected. 

These devices are called transition-edge sensors because they operate at the transition temperature between superconductor and nonsuperconductor. 

The sensors NIST builds must do two things. First, they must differentiate light emitted by the cosmic microwave background from that emitted by foreground objects, such as dust in our own galaxy. To do this, scientists take advantage of the fact that the spectrum of light (the light intensity at a given wavelength) from the background is distinct from that of foregrounds. By using different materials and designs, scientists can tailor transition-edge sensors to detect multiple colors and build maps of the sky that enable them to remove foregrounds and zero in on light from the cosmic microwave background. 

These sensors can also measure microwave light waves’ intensity and geometric orientation, or polarization. Cosmic inflation, if it happened, would have left unmistakable patterns in the intensity and polarization of the background light. These patterns, if they exist, are so faint that decades’ worth of observations have failed to find them. That has spurred the development of new sensors that, when assembled into arrays of hundreds or even thousands, combine precise measurements and collection efficiency not possible with other technologies.

NIST’s work on cosmic microwave background measurement goes beyond bespoke sensors. NIST-designed amplifiers help boost the strength of detected signals, and NIST’s unique electronics combine data from large into a small number of wires — a technique called multiplexing — to avoid overheating the delicate sensors. NIST scientists have also found innovative ways to package sensors and electronics to minimize measurement errors.

The Telescopes

NIST has provided transition-edge sensors and electronics for numerous cosmic microwave background telescopes and experiments. Some of these missions have completed operations; others are currently observing. NIST also plans to supply several future observatories. The 10 observatories and telescopes that NIST has supplied or will supply comprise most of the world’s past, present and future searches for gravitational wave imprints in the cosmic microwave background.

• Ali CMB Polarization Telescope (AliCPT)
• BICEP and KECK Array
• Cosmology Large Angular Scale Surveyor (CLASS)
• LiteBIRD
• Simons Array/POLARBEAR
• Simons Observatory
• South Pole Telescope
• SPIDER
• Taurus