Event Highlights

10 Things We Learned About the Universe from John Mulchaey’s Neighborhood Lecture

schedule 5
Astronomy & Astrophysics
Ten takeaways from "The New Golden Age of Astronomy: Hubble’s Universe Today" celebrating the 100th anniversary of Edwin Hubble’s groundbreaking discovery that our galaxy isn’t alone in the universe.
Date
Scientist
Dr. John Mulchaey
Division
Earth & Planets Laboratory
John M Points to Galaxy.jpg

Carnegie Science kicked off its 2025 Neighborhood Lecture Series with a cosmic deep dive from Carnegie Science President, John Mulchaey. His talk, The New Golden Age of Astronomy: Hubble’s Universe Today, celebrated the 100th anniversary of Edwin Hubble’s groundbreaking discovery that our galaxy isn’t alone in the universe—and offered a whirlwind tour of everything we’ve learned since.

Here are ten of the most mind-expanding takeaways from his lecture. 


🎥 Watch the full lecture:

1. Edwin Hubble Didn’t Just Discover a Galaxy—He Discovered the Universe

VAR! Plate
Carnegie astronomer Edwin P. Hubble took a plate of the Andromeda Galaxy (Messier 31) with the Hooker 100-inch telescope of the Mount Wilson Observatory. This plate is famous for having led to his discovery of the first Cepheid variable star in M31 (VAR!) which established beyond any doubt that M31 was a separate galaxy from our own.


In 1925, while working as a Carnegie astronomer at Mount Wilson Observatory, Edwin Hubble made a discovery that would forever change our understanding of the cosmos. By measuring the distance to one of the mysterious "spiral nebulae” that were vexing so many astronomers at the time, he proved this fuzzy-looking object was actually a whole separate galaxy—revealing that the Milky Way wasn’t the entire universe. This groundbreaking work laid the foundation for modern cosmology and cemented Carnegie Science’s pivotal role in astronomical discovery.

2. There Could Be a Trillion Planets in the Milky Way

JWST Image of a foreground cluster gravitationally lensing background galaxies.
Webb's First Deep Field (pictured) covers a tiny part of the sky and shows thousands of galaxies. This image focuses on the galaxy cluster SMACS 0723, which is captured as it appeared 4.6 billion years ago. Image: NASA


Thanks to decades of exoplanet discoveries, astronomers now estimate that most stars host planetary systems. We’ve identified nearly 6,000 exoplanets (and counting)! But that’s just the beginning. “We now think there could be a trillion planets in our galaxy alone,” Mulchaey noted. “Even if that estimate is off by a factor of ten, it’s still a huge number—and this is just one galaxy!”

With up to two trillion galaxies in the universe, we’ll let you do the math. 

3. Dark Matter Makes Up 90 percent of a Galaxy’s Mass

Rubin Dark Matter Graphic.png
This chart shows the expected rotational speed of stars at the edge of the Andromeda galaxy versus the observed rotational speed that Vera Rubin and her colleague Kent Ford observed. This finding was the first hard evidence for dark matter.


Invisible and mysterious, dark matter shapes galaxies from the inside out. Mulchaey emphasized Carnegie astronomer Vera Rubin’s landmark work, which showed stars in spiral galaxies move far faster around the galaxy’s center than visible matter alone can explain. While we still don’t know what dark matter is, we know it’s there—and it accounts for up to 85 percent of the matter in our universe.

4. Every Big Galaxy Has a Supermassive Black Hole

Black Hole.png
A supermassive black hole is one million to one billion times the mass of the Sun.


Black holes aren't just exotic oddities. They're central engines of galaxy evolution. Nearly every large galaxy, including our own, contains a supermassive black hole at its core. These cosmic giants can weigh millions or even billions of times more than our Sun—and light up as brilliant quasars when actively consuming matter.

5. We’ve Watched Stars Orbit an Invisible Object

The UCLA Galactic Center Group has been tracking specific stars orbiting the proposed black hole at the center of our Galaxy for 20 years using 2.2 micron images taken at Keck Telescopes on Mauna Kea in Hawaii. These stellar orbits, and a simple application of Kepler’s Laws of motion, provide the best evidence yet for a supermassive black hole, which has a mass 4 million times the mass of the Sun.
These stellar orbits, and a simple application of Kepler’s Laws of motion, provide the best evidence yet for a supermassive black hole at the center of our galaxy, which has a mass 4 million times the mass of the Sun. Image: UCLA Galactic Center Group.


Thanks to 25 years of careful observation, astronomers have literally watched stars orbiting a blank spot at the center of the Milky Way. The only explanation? A black hole. “This is the most direct evidence we have,” Mulchaey said, referencing work that earned a Nobel Prize in 2020.

6. Galaxies Collide—and So Will Ours

Galaxies interacting and merging to form a single galaxy
All of these galaxies are in the process of merging.


Galaxies aren’t static. They merge, interact, and reshape one another. The Milky Way is on a collision course with the Andromeda Galaxy in about 5 billion years. When that happens, even their central black holes will merge.

7. The Universe’s Expansion Is Speeding Up

Hubble showed us the universe is expanding, but recent measurements reveal that it’s accelerating. “Not only is it expanding. It's actually expanding at a faster rate in time. This is due, we think, to a mysterious force called the dark energy,” said Mulchaey.

8. We’re Living in the Golden Age of Telescopes

Next Generation of Telescopes
The next generation of big ground-based telescopes are so massive that they will capture even more detailed images than JWST when you correct for the atmosphere.


Mulchaey highlighted how JWST is revealing galaxies that formed just a few hundred million years after the Big Bang—breaking its own distance records again and again. Upcoming tools like the Vera Rubin Observatory, Nancy Grace Roman Space Telescope, and the Giant Magellan Telescope will push the limits even further, opening new windows into the early universe and the potential for life beyond Earth.

“These telescopes are going to let us study the distant universe the way we study the nearby universe,” Mulchaey said. “And the most exciting discoveries will be the ones we haven’t even imagined yet.”

9. Big Telescopes Need Big Support

The future of American astronomy, Mulchaey emphasized, depends on sustained investment in these game-changing tools. “Unfortunately, the future of American astronomy really could be quite in the balance if these telescopes don’t get built,” he said. “Basic research, which is what we do at Carnegie, is a great investment in the U.S. economy. For every dollar we spend, we make something like three dollars back.”

Help fund astronomy research.

10. The Universe Still Has Surprises for Us

Despite all our progress, many of the universe’s biggest questions—about dark energy, galaxy formation, and the origins of life—remain unsolved. And that’s what makes astronomy so exciting.

“Every time we build a new telescope, the most interesting things it’ll discover are the things we have no idea about,” Mulchaey concluded. “The universe always has surprises for us.”