When it comes to finding antonyms for the phrase "cooling down," we could look to words such as heating up, warming, or igniting. All three of these concepts connote an increase in temperature, whereas cooling down suggests a decrease in heat. Other antonyms for cooling down could include intensifying, inflaming, or firing up - all of which indicate an increase in activity or fervor rather than a calming or relaxing effect. However, it's important to note that the phrase "cooling down" doesn't necessarily have to refer only to temperature - it can also mean a reduction in emotions, agitation, or tension. For these definitions, antonyms such as ramping up, energizing, or stimulating could be used.