Private equity gained new power and responsibility as a direct result of the 2008 crisis. As cities and towns nationwide struggled to pay for basics like public infrastructure and ambulance services, private equity stepped in. At the same time, as banks scaled back their mortgage operations after the crisis, private equity firms — which face lighter regulation than banks, and none of their rainy-day capital requirements — moved in there as well.

The power shift has happened with relatively little scrutiny, even as federal authorities have tightened rules for banks. Unlike banks, which take deposits and borrow from the government, private equity firms invest money from wealthy individuals and pension funds desperate for returns at a time of historically low interest rates.

Since the 2008 financial crisis, private equity firms have gone from managing $1 trillion to managing $4.3 trillion — more than the value of Germany’s gross domestic product — according to the advisory firm Triago. Retirement nest eggs are fueling the growth and sharing in private equity’s risks and returns: Nearly half of private equity’s invested assets come from pensions.

“There is private equity — a lot of it — and it’s happening everywhere,” said Vikram Pandit, a former Citigroup chief executive who is now head of the Orogen Group, which invests in financial businesses. Across the financial landscape, he said, “New champions will emerge.”

Warburg Pincus, Kohlberg Kravis Roberts & Company, and other major private equity firms have invested in emergency services, a business that routinely holds the lives of customers in its hands. While this represents one small corner of private equity, which traditionally used debt to seize underperforming companies, it captures the industry’s newfound pervasiveness.

K.K.R. — a firm memorialized in “Barbarians at the Gate,” a book that chronicled a defining 1980s Wall Street deal — also invested in public water services. Blackstone is now America’s largest landlord of rental houses. And in the mortgage industry, until recently the province of banks, the Fortress Investment Group controls a huge bill collector.

In many of the fields where private equity now operates, it has not necessarily performed better or worse than the banks and governments it replaced. In some cases it financed projects that others wouldn’t fund and provided crucial public services, including emergency care. And because these firms do not rely on the government for loans, and are much smaller than Wall Street banks, they pose far less risk to the broader economy.