The MACHO Survey, led by Lawrence Livermore in the 1990s, sought to test whether dark matter was composed of massive compact halo objects (MACHOs).
A brief history of dark matter
Since the 1960s, astronomical evidence has suggested that an invisible, “dark” matter surrounds and permeates the disks of the Milky Way and other spiral galaxies like it. The 1980s saw a raging debate about the nature of dark matter. One notion was that it could consist of hypothetical elementary particles not yet detected (axions, massive neutrinos, or weakly interacting massive particles). An alternative idea was that dark matter could be made of massive objects such as brown dwarfs, neutron stars, or black holes (collectively known as MACHOs).
In 1986, Bohdan Paczynski suggested one method to identify MACHO dark matter through gravitational microlensing. A microlensing event happens when a MACHO passes through an observer’s line of sight to an ordinary, luminous star. The gravitational presence of the MACHO bends the light from the star, and, acting like a lens, causes a temporary apparent increase in the brightness of the star. However, the alignment required for this to occur is rare—at any given moment, only one star in a few million experiences a detectable microlensing event. At the time, no one had come close to observing the necessary millions of resolved stars regularly each night, let alone processing the amount of data required to find rare microlensing events.
A collaborative search for dark matter
While exploring additional ways to apply a new LLNL-developed imaging technology, a group of Lawrence Livermore physicists, led by Charles Alcock, realized it was technically feasible to observe and analyze the number of stars needed to detect a microlensing event. The technology’s charged-coupled-device (CCD) sensors, when combined with powerful parallel processing computers, could quickly produce, process, and analyze many thousands of images. Running with this idea, the team launched the MACHO project in 1989 with funding from the Laboratory Directed Research and Development (LDRD) program.
In the process of launching the program, it became apparent that the project needed more funding and more people. A year after launch, the Livermore researchers teamed up with others from the University of California’s Center for Particle Astrophysics. From there, the search began for a dedicated site to build a telescope. One requirement for the site was the ability to view the Megellanic Clouds, which are only visible from the southern hemisphere. Serendipitously, researchers at the Mount Stromlo and Siding Spring Observatories from the Australian National University became collaborators and offered to dedicate their 50-inch reflecting telescope (the Great Melbourne Telescope) to the MACHO project for four years.
Overcoming challenges
Assembling the MACHO team and getting access to a dedicated telescope was just the start; the team still had to overcome four key challenges.
First, they needed to observe as much of the sky in each exposure as possible, about 100 times more area than most cameras at the time. The Great Melbourne Telescope was a classic parabolic reflector that was good for narrow fields-of-view but produced poor image quality in the outer parts of the field. The team developed a novel lens system in the camera to enable good image quality across the wide field of view, a tradition continued with the Vera Rubin camera.
Second, they needed to determine if a star was getting brighter due a microlensing event or if it was just spontaneously varying in brightness. To address this challenge, the team created an imaging system with a dichroic beam splitter. This system enabled simultaneous observations of two different colors and highlighted the difference between the microlensing signal (achromatic) and standard variable stars (varied across their photometric spectrum).
Third, they needed to resolve the approximately 500,000 stars they were observing in their large field of view. To do this, each camera needed millions of pixels. The team developed and built the two largest CCD cameras seen in the astronomy world at the time, each with a total of 16 million pixels.
Fourth, they needed a way to analyze a record-breaking amount of astronomical data in real time. In the first three months of the MACHO survey, they collected more astronomical data than all preceding astronomical measurements combined. Novel algorithms and a dedicated multiprocessor computer reduced the raw data and searched through the approximately 50 million individual star measurements generated each night to find and characterize microlensing events.
Scientific significance
The MACHO survey spanned from 1992–2000. During this time, the team:
- Detected the first microlensing event.
- Detected the first parallax and binary microlensing events.
- Published the first real-time microlensing alert.
- Produced the first strong limits on MACHO dark matter, ruling out MACHOs in the Jupiter to 10 solar mass scale from making up more than about 10 percent of dark matter.
- Concluded that baryonic MACHOs smaller than 10 solar masses could not account for more than 40 percent of the total dark matter mass.
Beyond the many scientific results, the technology developed as a part of the MACHO project paved the way for many of today’s survey telescopes such as the Zwicky Transient Facility and the Vera Rubin Observatory Legacy Survey of Space and Time.
